Filling the Dent - Primer / Paint Issue

[LanceM71]

So I've made a few helmets with Smooth Cast 320 using the slush-cast method and had good luck priming and painting. However, on one of them, I filled the dent with Bondo and sanded flush. I've sprayed with primer, but the paint won't fully dry in some of the areas around the Bondo where the Smooth Cast was lightly sanded. I've tried 3 different primers, and have cleaned the areas with both alcohol and acetone (at separate times), but the primer will still not fully dry in those areas. It fully dries on the surface of the Bondo, and the areas away from where the Smooth Cast was sanded, but it seems like it's just the surfaces where the Bondo was sanded away from the Smooth Cast that are having the issue. It's not like it's orange peeling, or not bonding to the surface, it's just that it won't fully dry. I'm in California, so I don't think it's a temperature/humidity issue. Has anyone had this problem, and do they know of a solution?

 

[ToEleven]

Definitely a weird issue. What types/brands of primer did you try?

 

[WhyItMatterz]

Do you have any pictures?

 

[LanceM71]

Can't really see it in pics. I've tried Rust-Oleum Painter's Touch 2X Ultra Cover White Primer, Rust-Oleum Automotive Sandable Primer, and Sherwin Williams Bonding Primer. There's no problem with getting a good bond, just that it takes forever to dry in the spots that have been sanded. Doesn't seem to have anything to do with the Bondo, just wherever the urethane has been sanded. It looks like it does eventually dry, but in 3-5 days, as opposed to 15-20 minutes.

 

[PropReplicator2]

Could it have been an issue with adding too much catalyst or something to the resin when that was mixed? I'm not sure why it would cause you a problem after so many successful casts the past unless it's an issue with just that helmet.

 

[Grimstuff]

You may be putting them on too thickly, or just moisture or something with the resin yeah, but honestly many paints will take longer to cure on plastics and resins than they do other materials, also Rustuleum and Krylon primers are kinda ehh in my experience, especially their base ultra covers and such, they're pretty much just really flat regular plaint with the 48 hour+ cure times that entails. The only one I do really like is Rustoleum Universal Bonding Primer, sticks amazingly well to things.

This might sound weird, but the best sanding primer I've found is actually Harbor Freight's Sandable Primer. It goes on a little rough with it's cheap nozzle, but it legit dries amazingly fast, fully dry sandable in 30 mins. Makes a really fantastic quick working filling primer for dent work and such. I was a bit doubtfull when someone here suggested it too, but I'd definitely recommend trying it out if you need a quick cure primer.

Usually my priming method for poly helms is: degreasing soap wash with scotch pad, thin coat of Rustoleum universal bonding primer for optimal adhesion to the poly, then the Harbor Freight stuff for any scratch filling or general base coating. Then wet-sand and begin normal top-coats.