Shiny paint patches.

Hay All,
When I stick a panel to putty/plasticine/blue tack for spraying, I get a shiny patch corresponding to the contact with the putty under the panel.
Does anyone know the reason for this?
It polishes out evenly so not a big problem, except maybe an extra coat of clear required.

jk

Hay All,
When I stick a panel to putty/plasticine/blue tack for spraying, I get a shiny patch corresponding to the contact with the putty under the panel.
Does anyone know the reason for this?
It polishes out evenly so not a big problem, except maybe an extra coat of clear required.

jk
I get that also. I assumed that the glob of putty is like a heat sink. The cold paint warms up more where the putty is located and has more time to level and gloss out than on the thin plastic.

I've noticed this too, and would love to know "why?". Blu-Tac we need answers!

Thanks DD, the heat sink idea could be it but I think that the paint is thicker in the patch, so could it be that the paint is cooled quicker than the surrounding without the chance to level out?
jk

so could it be that the paint is cooled quicker than the surrounding without the chance to level out?


I would guess the opposite. Paint being sprayed is cooler than room temperature- it undergoes instantaneous cooling as it is sprayed (gas laws (http://en.wikipedia.org/wiki/Gas_laws) explain this). Thin plastic itself has very little thermal mass, so it is quickly cooled by the cold stream of paint. However an area that has greater thermal mass (like a spot having a glob of blu tac behind it) won't cool as much as the plastic around it. As a result, the paint on this spot is warmer than the paint elsewhere, and it flows better- allowing it to level out before drying.

It's pretty well known that warming paint improves it's flow characteristics. Submerging a can of spraypaint in hot water for a few minutes before use demonstrates this nicely. It still gets cooled as it is sprayed- but as it starts at a higher temp it is warmer on the surface being painted than it otherwise would be.

Ta MPWR, see the logic of what you say. It's getting pretty cool here, would a layer of blu tak keep me warmer?
Ta. jk

Possibly. Are you planning to paint yourself?

Hi Ehm it got nothing to do with heat etc...Sorry for burstin bubbles! However in plastic,rubber,blue tac...there's a substance that keep it from drying out and becomming brittle (Plasticizers) and keeps the plastic soft nice and shiny however this substance will even ruin a freshly painted surface on a real car if you put some plastic bag's on it too soon! The softer rubber, plastic etc is! The more Plasticizers are in there! and yes you polish it away...but if you are unlucky in a few months or years maybe these patches will re-appear! this stuff actually gets into your paint and even plastic!
Be carefull with Plasticizers!!! (please do your own research into Plasicizers! and the effects it can have on paint etc) i only learned about Plasticizers and their effects after i started painting real cars bikes and trucks

So be carefull with blue tac it can ruin your paint job!