To prepare your panel for bodyfiller you should consult the TDS for the product your using. Heavier fillers often suggest a coarser scratch from a grinder with 40-50 grit, finishing putty\’s like this one we will be using here suggest a finer scratch. We are removing the paint using 80 grit sandpaper on a dual action sander. Remove all of the paint from the dented area and about 1/2″ to 1″ beyond.
Some polyester putties do allow you to apply them over well cured old paintwork. Be aware that you can do this, but your adhesion now relies on the paint sticking to the metal. You are guaranteed good adhesion applying your polyester fillers to bare metal, old paintwork you are not. This only applies to finishing putties, heavy bodyfillers must be applied to bare metal.
Another exception, you can also apply an epoxy primer to the metal, allow it to dry and apply polyester products (including heavy duty fillers) over the epoxy. Paint manufacturers suggest better metal protection doing it this way, most filler manufacturers consider this unnecessary.
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