Polyester body filler - how long w/o paint?

[Popeye]

All,

How long can polyester body filler remain unpainted?

I have read that body filler is hygroscopic. If left unpainted it will absorb water in turn causing the metal underneath to rust.

Is this true?

If true, how long can filler remain unpainted? Days? Weeks? Months? Years? ...?

(I am planning out my resto, and trying to figure if I need to prime each panel asap. ...the next question is how long uncovered epoxy primer will keep...)

Many thanks,
Mike

 

[Steve]

I have heard that as well, and I have found from my own experiences in the UK that it is probably true. I would recommend a coat of primer as soon as the filler is smoothed and ready for the next step.

 

[dklawson]

Yes, the filler will pick up moisture... so will most sandable primers. In reality, you're only supposed to dry sand filler. Once primed you can wet sand it according to the recommendations for the primer you're using. IF you are working slowly on your project, you may want to look at the use of etch or epoxy primers below your high-build primer as these are more resistant to moisture and will allow you more time to complete your work. It's best to discuss this with your paint supplier and choose a series of components that are 100% compatible (which generally means they're all from one paint maker).

 

[Popeye]

Thanks guys. I am working with a local NAPA auto store that is a "platinum" distributor of PPG paints. They are great with advice and work with me. But sometimes they forget I am a hobbyist - who takes years to finish a product most body shops complete in a month... Most of their customers are local body shops.

Their advice was not to worry about moisture impregnating the filler, but thanks to you guys, they are not completely correct! Also thanks for the tip on epoxy primer.


Thanks again, the advice is much appreciated.
Mike

 

[dklawson]

I have used the PPG DCC products on my most recent paint job. It's a nice system. They have many primers compatible with the top coat. They should be able to advise you on a series of compatible components that will be good for your situation.

I'm curious though.... For years NAPA sold paint under its own name as well as Martin-Senour paints. I'm surprised that your local NAPA would sell a competing product like PPG.

 

[billspit]

I always used epoxy primer on bare metal before I put on the filler. That should help with moisture problems alot. Then I primed over it when finished sanding the filler. If you keep the car inside and don't live in a humid environment, I think you may be OK.

Epoxy primer will last a long time before top coating, but you will need to sand it if you let it sit over a week before top coating. It gets too hard to hold a top coat otherwise.

Typically moisture will go through a primer surfacer but not a primer sealer.