what is meant by using "Smoothing compounds" to use between sanding a vehicle before painting it?


read an answer on one of my previous questions about sanding a vehicle before painting with a Random Orbital sander and if it is best to use instead of a "Straight back and forth vibrating sander" and was told that the random orbital sander is better as it gives a better, finer finish and was also told this

"they wet sand the primer coats after they do any bodywork or use smoothing compounds before they do a majority of the sanding. "

1 Answer

  • 1 year ago
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    A smoothing compound is a "plastic" polyester or glazing putty that is used to fill imperfections and small dents where the panel doesn't need much metalwork or doing said metalwork can lead to more issues. Most people use the generic "Bondo" identifier though there are much better choices than that commercial brand. Filler compound is another name it can be called BUT compound isn't meant to fill deep dents where real dent repair should be used nor over large areas unless the person knows what they are doing (skim coating for example).

    I learned my nomenclature from doing aircraft/airframe repair before doing any car bodywork or painting, but most of the items inter-relate.

    Bondo and other fillers are brittle and have almost no tensile strength, can absorb moisture and most can't be applied over some primers/sealers, while most do bond to epoxy primers. That gives the option to prime a vehicle or panel with epoxy to prevent flash rusting while still being able to smooth out the needed areas later with just a roughing up.

    Bodywork and painting is a multi-step process with repairs, metal prep/rust abatement, build primer coats, sometimes an added primer/sealer coat on top of the build primers before you can apply color or base/clear.

    Have to sand out the build coats and use a guide coat to spot low areas and watch out for cutting through to metal on high spots.

    I'm doing a 55 F-100 down to bare metal for the most part so I have to do all the processes including welding in rust repair panels, popping a few dents and fabricating some stuff.

    Can do dry sanding for the primer, and paints, I just don't like the dust. You do want to dry sand filler and often before it is fully cured depending on which ones. Also, there are certain ways to sand, normally you do a crosshatch process where you rotate 90 degrees while sanding to get full effectiveness and lessen the tendency to gouge or over-sand. Have to use light pressure and let the grit do its job or again you can get uneven sanding.

    A DA sander provides a similar process as the crosshatch sanding, but it can still cause swirls and using a DA wrong can make for a long day.

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