Filler before or after primer

[JohnU]

So, I did a google site search and didn’t find a really definitive answer to this question. Most seem to use filler before and a few use it after priming and sanding when the need is visible. I guess I’m looking for an advantage over either for the best stable results. Thanks in advance!

[gman]

It will depend on the filler. Some solvent-based fillers will dissolve primer, while some 2 part catalyzed fillers are perfectly safe over primer and will not lift it if applied over top.

If you are going to use something like Squadron putty, do your body work on bare plastic. Let the putty gas out and fully set/cure, then prime. If you are going to use a 2 part polyester glazing putty, it can be applied over roughed up plastic or roughed up primer. Re-prime after bodywork to seal it before color coats are applied.

Edited June 29, 2019 by gman

[Snake45]

Before. And then after if necessary (followed by more primer). Repeat as necessary. It ain't rocket surgery. 

[Mark]

I'll agree with before, and I apply primer with a brush (a cheap one will do) ONLY on the worked areas.  No blasting the primer on with a rattle can until those areas are right.  That minimizes buildup on other areas, particularly those with fine detail like scripts or exposed hinges.

[Rider]

Before, during, after, I have never had an issue with shrinking/cracking. 

[espo]

I try and do as much body correction as possible before the first coat of primer. After sanding everything as smooth as possible it's time for the first light coat of primer. Follow this with some very light sanding to show any other imperfections that couldn't be seen before priming. This should leave the body surface with almost no primer showing.  This should bring out any surface that you couldn't see before. You may find some areas that will need additional filler or just sanding and leveling. I suggest a very light primer coat before going to color coats. I will use sanding pads in the mid to high 2000 range as this  gives the color coats something to adhere to and you know that the surface is as smooth as possible.    

[JohnU]

All righty then, I think I have the information I need now. This is the first time I’m using filler to fix some minor body sink holes. Usually I just block sand the plastic down and highlight the low areas with a pencil and repeat until it looks good to me. The current project I’m working on have some pretty deep ones that will require more attention. Thanks for the feedback!