Does Bondo require special primer?

Ray IN

Member
I am using Magic primer and paint with hardener on
some sheet metal parts. I was told that the bondo
will eventually show through or cause rings if I do
not use some other primer over it. Help because I
have never painted over Bondo before.
 
The best primer is epoxy primer. But the rings are caused by not sanding the bondo smooth enough. A lousy job will show through any primer/topcoat. Anyway you should use a surfacer over the primer to get the best finish.
 
The rings are caused by just primering over bondo with adhesion primer as apposed to using adhesion primer( epoxy) and then 3 or 4 coats of "build primer" or primer surfacer which will build up a thick (3-4 mil) layer to fill in minor imperfections and make a uniform and flat surface once block sanded. You can't get bondo perfect enough to not show without doing this. After yo get your surfacer on then lightly fog a little black from a spray bomb over it. After a day or two dry (depending on if you use a catylized surfacer) use a hose or bucket and sponge and wrap some black wet-or-dry 320-400 grit sandpaper around a hard rubber sanding block and sand until ALL the black is gone. It will now be flat and ready for paint. If you have to go down to metal/bondo to get all the black then dry and re-prime that area and repeat. If you want it smooth that's what you need to do. No rings.
 
Thank you to all. I think I am finally getting it! Is the epoxy primer and surfacer available at most auto parts stores?
 
Actually I have done a fair amount of painting over bondo and have yet to buy any epoxy primer and do not have the bondo show rings thru. In 1957 I painted a car that I didn't get enough primer over sanded out stone chips and had a do over. I haven't had that problem since.
 
Sometimes bondo drinks up primer and is a little difficult to cover but once you get it covered it will be fine. A primer/sealer will cover it easier.
 
Yes, when you prime over prepared metal that has had some defect repairs, you will see some of the defects after it cures, but that is why you use sandable primer and block sand before you apply the color coats. That's part of the preparation. You block sand till the defects disappear. You might have to apply more primer and repeat until you have filled all the defects.
 

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