Smooth cast 300 with vallejo primer gets soft


New Member
Hi everyone,

my english is not the best but I hope you understand me. I'm still very new in the modeling and painting and try out a lot. Currently I'm using Smoothcast300 and that has worked quite well so far. Now I try to paint my parts with Aribrush and Vallejo acrylic paints. For priming I use Vallejo Primers Grey 74.601 and as compressor I use Airbrush Compressor Set Compressed Air Spray Gun Includes 1x Piston Compressor & 11 Pcs Airbrush Spray Guns Set (with Storage Case

My problem is that after priming, the part becomes soft or sticky in some areas, and I don't know what is causing it.

I don't think it's due to drying time because it stays that way even after several days.

I suspect it is either the component or in some areas, the primer or the compressor because it sometimes blows out a lot of water.

Have any of you experienced this before and do you have a solution for me?

Thank you in advance.
zorro is it possible that there's any of your release agent from casting still on the part causing a reaction with the paint? As long as the Smoothcast 300 is fully cured it shouldn't be reacting negatively with an acrylic primer.
Hi TurboCharizard
thanks for your answer.

I have also thought that I may have taken a wrong mixing ratio but that this then only occurs in some places makes no sense to me. Because I let the component dry for days and also cleaned a second component with soap, it makes it even more confusing for me.
It is hard for me to understand why it is only in some places.
additional info
I have two parts. I trim them to fit together and use Smoothcast as a "glue". can this be a reason and if so how do you glue two parts together?

There are a few more things I could do.
Skip the primer (easiest way, but I don't know if it makes a difference if I use Vallejo primer or Vallejo acrylic paint as a first layer).
Spray the part with finisher first and then paint it (the order doesn't make sense but maybe I'll get a neutral layer)
or simply pour epoxy resin over the whole thing and get a hard outer shell (but I would really not want to do that).
maybe the gluing is the problem at least if you want to paint it then. How can I, for example, a skull that I have split into two parts put back together. Can't I apply Smoothcast to the edges, glue them together and sand the edge? :unsure:
Short update from me. Drying the wet areas with a hairdryer only makes things worse. :cry:
I'm now trying to seal the whole thing with epoxy resin and then possibly sand it again to achieve a matte surface. Additionally, I will create a new silicone mold and cast the part as a single piece :confused:

thats how it looks like. I don't now what I'm doing wrong. The problem is you only see it after painting and if there is something like that you can start again.
I read something that I'm not the only one with problems with smooth cast 300 and 320 should be better or the primer is the mistake and I should use one which is normally used for cars
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When I make skulls, I use a Rustoleum matte (Chalked series) tan primer, then cover it in a layer of Rustoleum Chalked varnish to protect the primer. I then weather it with a water based oil paint, and after leaving that to dry for a day, give it another good layer of Chalked varnish.

The only time I've had a problem with the paints was when the primer turned a slightly bright green yellow colour after varnishing; this turned out to be the filament (Bambu Green PLA) that caused the problem, even after a layer of (cheap) resin. Using my normal filament (eSun grey PLA+) I have no issues.

Are you able to get your hands on other primers, such as Rustoleum?
Thank you PlanetAlexander for your input!
Unfortunately I have not found any Rustoleum on Amazon. because I use a polyurethane casting resin, I also do not know if that is the best primer. Because I have just started airbrushing, I took what had good reviews on amazon and found the Vallejo. But I think it is used more for miniature models and not for my application. If I do not find other info, I'll just test if there is a primer that fits better
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