two separate components are diluted and mixed into a hydrofluoric acid generating solution or reactive mixture near the location ( e . g ., around the time ) of application to a surface to be brightened . the dilution and mixing of the two components are done on site ( near the location of use ) before use ( e . g ., the components are preferably mixed on the same day as they are applied , preferably within hours of application , or even within minutes of application to the surface to be cleaned and even in less than 15 seconds before application ), at the time of application to the surface ( e . g ., in a single head nozzle or multiple head nozzle ), or directly on the surface to be brightened ( e . g ., first one component applied and then the second component applied or both at the same time ). the two essential ingredients which must be present are a fluoride source and an acid solution . the invention may be practiced as simply as a ) combining the fluoride source and the acid solution on site before spraying , b ) combining the two sources in moving streams immediately before , during or after they pass through a spray nozzle , or c ) combining the two components on the surface to be brightened in separate or contemporaneous applications from two separate streams . the fluoride source should be any water - wettable or water - soluble or acid - soluble fluoride salt or solution , such as for example , potassium fluoride , sodium fluoride , lithium fluoride , ammonium fluoride , ammonium bifluoride , sodium silicofluoride , calcium fluoride , magnesium fluoride , alkali metal fluorides and difluorides generally , alkaline metal fluorides and difluorides generally , and any other compound which forms hydrofluoric acid when combined with sulfuric , nitric , and / or phosphoric acid . the fluoride source may be provided as a solid or flowable powder to be sprayed onto the surface or mixed in a stream with the acid , but is preferably provided as a solution of the fluoride material , such as from 2 to 50 % by weight fluoride salt , preferably 5 to 50 % by weight fluoride salt , more preferably from 10 to 50 % by weight fluoride salt on a weight basis of the solution . the acid should be a relatively strong acid , and one which in solution with the particularly selected fluoride source will generate hydrofluoric acid from that fluoride source . for example , sulfuric acid , phosphoric acid , sulfinic acid , nitric acid , phosphonic acid , hydrochloric acid , sulfamic acid and mixtures thereof will work . the acid is supplied as an aqueous solution with , for example , from 5 to 85 % by weight acid , more often from 10 to 85 % by weight acid , and most often between about 15 and 85 % by weight acid in the solution . the diluted solution of fluoride ion can comprise about 0 . 01 to about 1 . 0 % by weight fluoride salt , and can contain about 0 . 1 to about 0 . 5 % by weight fluoride salt . the diluted solution of acid can comprise about 0 . 02 to about 2 . 0 % by weight acid , and can comprise about 0 . 05 to about 1 . 0 % by weight acid . at least one of the diluted solution of fluoride ion and the diluted solution of acid can comprise a surfactant in an amount of about 0 . 001 to 0 . 2 % by weight of the solution in which it is present . at least one of the diluted solution of fluoride ion and the diluted solution of acid can comprise a quaternary ammonium compound in an amount of about 0 . 001 to 0 . 2 % by weight of the solution in which it is present . additional , optional or preferred components include , chealating agents , surfactants , metal ions , foam supressants ( carbamides , dicyanoamides , triazines such as 2 , 4 , 6 - triamino - s - triazine ), thickening agents ( e . g . methylcellulose , hydroxymethylcellulose , synthetic resins , silica ), organic acids ( such as oxalic acid ), sulfonated polymers and the like and the basic materials in the fluoride source component ( such as naoh , koh , ammonium compounds , and the like ). these additives may be used in the relative concentrations appropriate for the particular ingredients and use . for example , the surfactants may be used in amounts of from 0 . 1 to 20 % by weight of the concentrate ( more generally from 1 to 12 % by weight of the concentrate ), and the foam suppressants may be used in amounts of from 0 . 05 to 7 % by weight of the concentrate . the metal ions may be useful at much lower concentrations as for example from 0 . 1 to 2 % by weight of the concentrate , although wider ranges for each component may be selected as needed , without limitation by these generally described ranges . each of these may be absent from use solutions . the practice of the invention therefore includes a method of treating a surface of a metal comprising the steps of mixing an aqueous solution comprising a source of fluoride ion and an acid of sufficient strength to form hydrofluoric acid when mixed with the source of fluoride ion , and applying the solution ( s ), as for example by spraying the solution ( s ), onto a surface of metal to be treated . the mixing of the aqueous solution may be performed by mixing a first solution comprising a dissolved , water - soluble source of fluoride ion with a second solution comprising the acid . the method may comprise the steps of separately advancing a first and a second solution towards a mixing zone , mixing the first solution and the second solution within the mixing zone in a continuous flow process to form an active hydrofluoric acid - generating solution , and spraying the active solution onto a surface of metal to be treated . the mixing zone may near a spray nozzle , such as immediately before a spray nozzle . the mixing zone may be a passive mixing zone ( e . g ., merely a tube into which both solutions are combined while moving towards a nozzle ) or an active mixing zone where turbulence , agitation , or shear forces are provided to assure mixing of the solutions . the mixing zone may also be a temporary holding tank , from which the mixed solution is drawn for spraying . the device for dilution and mixing may be any fluid direction mechanism which can combine at least two streams together before or at the time of emitting solution ( s ) from the mechanism , such as an injection pump , which pumps the concentrates a and b and injects them into a water stream ( s ) to make solution a , b or a + b . the device may be an eductor or venturi , which aspirates the concentrates and mixes them into a water stream ( s ) to make solution a , b or a + b . in the case where a and b are diluted separately , the two streams are combined in the mixing zone before spraying onto a surface to be treated . fig1 shows examples of how an eductor may be used to dilute and mix one component at a time by aspiration ( fig1 a ), or two components simultaneously ( fig1 b ). fig2 ( a - d ) show a chart of the various ways in which liquid concentrates a and b may be diluted and mixed for application to a surface to be treated . the various mixing methods include : a ) mixing at least two streams , for example each of streams a ) and b ) which comprise a source of fluoride ion and a source of an acid of sufficient strength to form hydrofluoric acid when mixed with the source of fluoride ion . these streams may be provided independently with separate streams of water to form diluted solutions of the concentrates a and b , then combining the two diluted solutions into an active ready - for - use ( or partially diluted ) solution in line or through an optional holding tank , then directing the active solution to an applicator ( e . g ., a spray nozzle ). the mixing device may be an injection pump , an aspirator , a venturi , an eductor or any other convenient mixing device . the product concentration may be further diluted in line or in the optional holding tank . b ) mixing each of streams a ) and b ) which comprise a source of fluoride ion and an acid of sufficient strength to form hydrofluoric acid when mixed with the source of fluoride ion , sequentially with a single stream of water to form an active ready - for - use ( or partially diluted ) solution , optionally storing the active solution in an optional holding tank , then directing the active solution to an applicator ( e . g ., a spray nozzle ). the mixing device may be an injection pump , an aspirator , a venturi or an eductor . the product concentration may be further diluted in line or in the optional holding tank . c ) mixing each of streams a ) and b ) which comprise a source of fluoride ion and an acid of sufficient strength to form hydrofluoric acid when mixed with the source of fluoride ion . these streams may be mixed simultaneously or subsequently with a single stream of water to form an active ready - for - use ( or partially diluted ) solution , optionally storing the active solution in an optional holding tank , then directing the active solution to an applicator ( e . g ., a spray nozzle ). the mixing device may be an injection pump , an aspirator , a venturi , an eductor or any other mixing device which can combine the necessary materials . the product concentration may be further diluted in line or in the optional holding tank . d ) mixing each of streams a ) and b ) which comprise a source of fluoride ion and an acid of sufficient strength to form hydrofluoric acid when mixed with the source of fluoride ion . the streams may be provided independently with separate streams of water to form diluted solutions of the concentrates a and b , then directing the active solutions to two applicators ( e . g ., a spray nozzle ) for simultaneous spraying and mixing on the surface to be treated . the mixing device may be an injection pump , an aspirator , a venturi , an eductor or other mixing device . when concentrate a or b are solid blocks , the schemes of diluting and mixing shown in fig2 a , 2 b and 2 d may be modified as follows . instead of injecting or educting a liquid concentrate into an aqueous stream , a nozzle is used to spray an aqueous stream ( e . g ., water ) on a solid concentrate block to form a diluted solution by erosion or dissolution of the solid . this is shown in fig3 . this method may be used to form either a or b or both aqueous streams before proceeding toward the holding tank . in this particular format of hydrofluoric acid application , the holding tank may be needed for the solid application ( as particles of material may be carried by the aqueous streams . an applicator device such as a nozzle may then be used to apply the solution from the storage tank . one particularly useful scheme is to form an acid stream by forming an aqueous acid stream , and spraying the acid stream on a solid concentrate comprising a source of fluoride ion , which has a low water solubility but a high acid solubility . this may or may not require a holding tank . when concentrate a or b or both are powders , the powder component may be either sprayed with an aqueous stream as described above for a solid block ( a supporting screen or other support surface may be used here ) or the powder may be added directly into the optional holding tank . the invention may also comprise a kit for the treating of a surface of a metal comprising two separate containers , a first container housing a source of fluoride ion ( aqueous solution or a powder ), and a second container comprising an aqueous solution of an acid having sufficient strength to form hydrofluoric acid when mixed with the source of fluoride ion , at least one of the said aqueous solutions having a surfactant dissolved therein . the system may be free of phosphoric acid . these and other aspects of the invention will be further shown and described in the following , non - limiting examples , which are not in any way intended to limit the scope or equivalents useful in the practice of the invention as broadly described . the basic methodology used in these examples selects a 1 × 3 inch ( 2 . 5 by 7 . 6 cm ) aluminum coupon ( alloy 5052 ) and immerses them in a 1 . 0 % naoh / 2 . 0 % versene 100 ( chelant ) solution . the coupons are then vigorously rubbed with a soft , non - abrasive sponge , then rinsed before immersing them in an acetone bath . the coupons were then air - dried at room temperature , and their gloss values recorded at a sixty ( 60 ) degree angle using four ( 4 ) reading each time , for each coupon , in a gardner ( byk ) micro - tri - gloss gloss meter . two separate concentrates ( concentrates a and b ) were prepared as follows , with all units being as percentage by weight : solutions a and b were then prepared from concentrates a and b , respectively , by dilution and mixing into distilled water to give a final concentration of 0 . 25 % of the concentrates . the data sets used the following compositions as the brightening solution in practicing the method of the examples . set ( c ) comprised an equal parts mixture of solution ( a ) and solution ( b ). the two solutions were mixed and allowed to equilibrate for 15 minutes before being sprayed as a single stream of solution . set ( d ) was applied as two separate sprayers applied equal amounts of solutions ( a ) and ( b ) with mixing of the two solutions when the two sprays overlapped in the air and collided simultaneously or mixed ( on the aluminum coupon surface ) with the aluminum coupon surface . set ( e ) was sprayed from a single sprayer , using equal amounts of each of solution ( a ) and solution ( b ). the mixing was assumed to occur when the two solutions were drawn up symmetrically into a single tube after and concurrently when the solution exited the spray nozzle . set ( f ) used distilled water as a control . it was sprayed , as were all other solutions , using an aerosolizing spout . after spraying , each coupon was immediately laid horizontally and allowed to lay horizontal for twelve ( 12 ) minutes , after which each coupon was immersed twice in hot tap water , and then dunked into acetone . each coupon was air dried for six ( 6 ) minutes . the coupons were then again measured for gloss at a sixty ( 60 ) degree angle , and the data recorded . the difference in gloss before and after treatment was then calculated to provide a delta gloss value , reported in the table as avg . delta gloss . the results evidence the following conclusions . sets c , d and e which combined solutions ( a ) and ( b ) displayed better results than either individual set using only one of solutions ( a ) or ( b ) and set ( f ) where only water was used . the performance of set ( d ) where the solutions were believed to mix and react on the surface of the coupons was apparently higher than where the solutions were mixed in the nozzle ( e ) and where the solutions were premixed ( c ). this is particularly surprising in that one might infer from the respective data between sets ( c ) and ( e ) that there might be some equilibration time desired for complete formation of the hydrofluoric acid from the components , because the equilibrated solution was marginally ( possibly within the limits of experimental error ) higher than set ( e ) where the solutions mixed in the nozzle . the fact that some equilibration time might be desirable , and the fact that mixing of the separate solutions in air , in transit to the coupon surface , and / or on the coupon surface provided statistically significant improved results is definitely surprising . the main objective of safe handling of the active solution by forming the hydrofluoric acid on site shortly before applying the active solution to the surface to be treated is not performed at the expense of the effectiveness of the cleaning solutions . the data are tabulated in table 1 , showing on - surface / in - nozzle hf generation and aluminum brightening . the combination of solutions described above in the examples , with 5 - 30 % fluoride salt , 15 - 40 % sulfuric acid , and 0 % or 0 . 5 % to 10 % by weight of organic quaternary ammonium compounds has been found to be particularly useful . the compositions may consist essentially of those two or three basic ingredients and may have levels of other acids ( e . g ., less than 5 %, less than 3 %, less than 1 % or none ) such as nitric acid and phosphoric acid . in the practice of the present invention , where the mixing of the solutions is described as near the applicator or spray nozzle , this means that there is only a single conveying means ( tube or pipe ) in place between the point of mixing and the applicator , or that the mixing is actually effected after the spraying ( in the air in transit to the surface dr on the surface ). these and other non - limiting examples of the invention are provided herein .