Paint drying issues with Smooth-On Smooth-Cast 321 Resin?

[geek_on_parade]

Hello all! I'm new here and this is my first posting. I tried searching for an answer to this problem to no avail. Hopefully someone here knows something about this.

I created a mold of a prop out of Smooth-On OOMOO 25 and made four casts out of Smooth-On Smooth-Cast 321 Resin. The result was really good and, after prepping the casts, I primed with Rust-Oleum Automobile Primer; I allowed the primer a day to fully dry (even though it dries in <30 minutes). The next day, I painted the castings with Krylon "Fusion for Plastic" paint. Now, all seemed well and I allowed the casts to dry over night; again, this paint dries quickly (<30 minutes). It has been three days since I painted them and two of the four casts still have "tacky" paint on the ends that were also at the molds entry point (the highest point in the mold). All of the casting and painting was done on clear sunny days.

Has anyone else ever had a similar issue? Any solutions? It just seems a bit bizarre that I would only have this issue with paint not fully drying on two of the four and only at specific areas of the castings.

Any insight would be greatly appreciated.

[modelnutz]

How long did you let the resin post cure before you painted the castings?
Most Urethane resins require a minimum of 5 to 7 days to fully cure.

You may need to strip the paint from the castings and boil the castings to fully cure the 321.

[Alex Dumas]

How long did you let the resin post cure before you painted the castings?
Most Urethane resins require a minimum of 5 to 7 days to fully cure.

You may need to strip the paint from the castings and boil the castings to fully cure the 321.

Boil the casting? Interesting How much time?

Smoot-on suggests to post cure the castings in an oven (150°F/65°C) for 4 to 6 hours.

[Kylwell]

While Fusion for plastic is actually make for resin deck chairs & what I'e found that if it isn't shaken like LA in an earthquake it'll dry tacky. Crappy stuff if I do say so myself. Last time I used it, it took several months to fully cure.

Strip it off and start anew.

[Joseph Osborn]

Any chance that those two casting with paint problems were the first two castings from the mold? As we've discussed in the past couple of days, tin-based silicone have excess alcohol that will inhibit the cure of the resin for the first casting, maybe two castings. Unless you post-cure that rubber at 125F for a few hours, the mold will normally give you a bad finish on the first pull. Platinum silicone doesn't have this property-- it will make a perfect casting from the start without requiring post-cure heating.

[geek_on_parade]

Thanks for the replies!

The castings were allowed to cure only a few days as I filled air bubbles and prepped the surface for paint. I guess it wasn't enough time.

They were the first two castings from the mold; interesting. The first casting is actually more tacky than the second. The paint becomes less tacky everyday though. So, say I want to try the boiling; what would should be used to strip the paint that will not affect the resin?

Again, the issue is only at the area of the casting that is the highest point in the mold; could the alcohol have "floated" to the top prior to curing?

Another interesting note: I created a couple of other molds (smaller pieces) the same day and I went back to inspect those castings to find that the first casting has the same "tacky paint" issue as the larger castings. They were missed previously because of where the "highest point" was on those particular pieces.