Hi,

I'm putting a Matchbox Flower class corvette kit together, and I have used a fair bit of silicone rubber in the process. To cut down on vibration, the motor mount is glued to the base of the hull with silicone, and the maindeck/engine room superstructure is stiffened underneath with an aluminium bar, again secured to the deck with silicone.

The kit is, presumably, made of polystyrene, and I have used 2 mm styrene sheet for the maindeck. The silicone rubber is stuck behind this, so is obviously not on show. However, it has an effect!

I have just started to spray primer undercoats so that I can see imperfections more easily. I am using semi-matt car white, red and grey, which seem to adhere well to the polystyrene. Over the places which have silicone behind, the paint becomes more glossy, almost as if some oil had been dropped there. It looks as if the silicone is releasing oils which are passing through the 2mm of styrene.

I did not think that styrene was porous. Has anyone had this problem before?

Ahoy, Mate!
I can honestly say that I have NEVER run into this problem before, but I will also tell you that yes, many types of styrene is porous,my friend. . .I am far from a chemical analyst, but I can definitely agree with what you are experiencing as being totally possible. . .and I don't know of any sealers that may combat this problem. . .definitely going to try to come up with an answer, though!
~Gunny

I suspect the difference in gloss is due to the density of the two surfaces. The primer, if solvent based, attacks the styrene and allows penetration and adhesion. The primer on the silicone is for the most part on the surface exhibiting more gloss. Since one of the reasons to use a primer is to uniform different substrates so the top coat will not have different gloss levels, you may not have a problem when you top coat. This information is strictly based on paint application technology. I have not experienced your problem first hand.

Don

Ah, no, Don - I have obviously not explained the problem well enough.

I am not spraying paint onto a styrene surface and a silicone surface, and expecting the results to look the same.

I am spraying a prepared, flat styrene surface. BEHIND this surface, in some places, is some silicone rubber. What I have found is that where the silicone touches the styrene behind, the corresponding front of the styrene behaves as if oils were coming through.

There is no joint in the styrene - it is 2mm solid plastic. And there is no doubt that the glossy areas correspond to the silicone attachments behind the styrene.

The silicone was well cured for a few weeks before I began painting, and I cleaned the surface with meths before spraying.

OIC. It does seem that the silicone has migrated through the styrene. A test for adhesion may confirm it. Place a piece of tape over a glossy area. It the primer comes off easily when the tape is removed, there is probably silicone on the surface. Silicone will not allow paint to adhere. As far as solving the problem I am afraid I do not have an answer. Silicones are virtually impossible to remove. Cleaning the surface usually just moves them to another area. A 50/50 solution of acetone and water may help the situation if the surface is not too badly contaminated.

Don

Thanx for the response. The tape idea is a good one and I shall try it - at the moment the paint seems to be holding on OK.

If something is coming through cleaning the surface probably won't help - I have a large block of silicone behind the styrene which will presumably keep sending stuff through.

Interestingly, I had actually shifted the motor mounts rearwards before painting and removed much of the old silicone mount (though not all). This silicone has obviously had longer to cure, about 2 months. There appears to be no mark in front of the old silicone. I wonder if the oils stop coming out after a certain time?

I presume this effect will be worse for wood - perhaps people should be warned to keep silicone well away from paint!