Prip’s Flux

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Last updated: 7/28/17

Prip’s Flux is a fire scale preventative.  It will not always prevent fire scale – but, it helps a bunch.  See my related web pages for more information on fire scale – especially:  Oxidation, Flux and Fire Scale Prevention (should add Fire Stain too!).

Safety – Wear a particulate (N95) mask when measuring and pouring the chemicals as they are powders and the dust will enter your lungs.

The Recipe:


1 quart or 4 cups of  water

½ cup or 4.23 ounces or 120 grams boric acid (available at pharmacies)

2.822 ounces or 1/3 cup or 80 grams T.S.P.  Ensure that your box of T.S.P. notes that it contains trisodium phosphate.  (Available at hardware stores too). Moderate to strong alkaline product.  Moderate skin and severe eye irritant.  If ingested, can cause burns to internal organs. MSDS.

2.822 ounces or 1/3 cup or 80 grams borax – available at the grocery and probably, the hardware store.  I use 20 Mule Team Borax, which is 99% Borax and 1% trace elements.

This is generally a 3:2:2 mix with 3 being the boric acid and 2 the T.S.P. and Borax.

Boil mixture, stirring – adding more water if necessary, until all powders are dissolved.

It is important to spray this flux onto WARM metal.  So, heat briefly with the torch and spray until you have a fine layer of the mixture and there are no spots of bare metal visible.  You can apply your soldering flux and solder before spraying or after or not at all.

You can also dip the piece into the mixture or brush it on.  These methods are not ideal but, better than nothing.  When using the dipping method or brushing method, heat until the flux dries evenly.  Your goal is to just dry the flux and not to over or under heat it.  With overheating you will see it change color (yellow or brown color) with under heating, it will remain liquid in spots.  The Prip’s should be white when heated properly.

When the Prip’s is dry,  apply your soldering flux to the join being soldered and then the  solder.  Theoretically, you shouldn’t need soldering flux but, I often use it.

The flux will re-crystallize and can clog sprayers.  You can either re-heat the flux or use a mouth atomizer to aid in application.

An atomizer will work better than a trigger-type of spray bottle.  You want a fine spray.

Another method is to let the flux sit for  a few days, until  crystals begin to form.  Pour off the liquid into the spray container and leave the crystals behind in the storage container.  If you are using a spray bottle or an airbrush and the sprayer is clogged, transfer the mixture to another container and then thoroughly clean the spray container and spray mechanism with water – removing all crystals. Then pour the solution back in. If possible, store the spray mechanism separately so, that crystals don’t form in the hose or alternately, pour the unused portion of flux back into the storage container when you are done and rinse the sprayer well – give the spray mechanism a few pumps in clear water to rinse out any of the solution.  This will avoid crystal buildup and blockage.

You can read more about Prip’s flux in Peter W. Rowe’s article at Ganoksin. Also, Deborah E. Love Jemmott’s page.

Mouth Atomizer  Mouth Atomizer  To use the atomizer, check out this video:  by Jo Toye and ArtAlternatives (to see the atomizer in action).  These videos are about using the mouth atomizer with paint but the concept is the same.

Related web pages