Hello all...

I know that this topic has been covered previously, but I have some specific question I would like to ask about doing this.

A bit of history...
I am building a camper body for my truck and have chosen to do this with one off, destructive moulds and vinylester resin infusion. One part per mould then throw the mould away, so keeping the costs down as much as possible would be ideal.
My initial testing went pretty well, but the main problem I have encountered is sealing the MDF moulds so that they are vacuum tight. I have used numerous methods, but none have proved successful so far.
I am currently trialing different methods, based on what I have read here, in other places and what I have personally learnt so far.

The last part that I made was over a mould covered with numerous coats of 2K automotive polyurethane paint. Everything looked good, but it still leaked. I won't go into the fine detail here, but I chose to infuse with a vacuum of 10 in/hg, which I knew had a low chance of success. The end result was not as bad as I had expected, but there were definitely areas that had dry spots that needed to be repaired. Although this was a pain, it was not the biggest issue. When I looked at the mould I saw that resin/styrene had got under the paint and this had resulted in blistering (probably the main source of my leaks). Where the paint had lifted it looked like cleanly sanded MDF, so it was obvious that this paint had not keyed well at all. My first coat was definitely a thinned coat, so seeing this came as quite a surprise.

Anyway, what I need to do now is coat the mould in a way that this issue does not occur again. I am currently preparing some small test moulds with different surface treatments.
One is simply coated with epoxy resin (brushed on), the first layer being thinned with MEK at 15% by weight. Yesterday I also purchased some automotive 2K epoxy surfacer/sealer (Valspar VP50), but as yet I have not applied that to a test mould.
I chose the epoxy sealer/primer over normal 2K primer/surfacer as I felt it would key better. This is not my area of expertise and the guys in the paint store, although being helpful and knowledgeable in automotive painting techniques, knew little to nothing about sealing MDF for a successful resin infusion. This came as no real surprise, as they were not fibreglassing people.

The body will be post painted, so obtaining a mirror finish on my moulds is not a priority. All I require is a sealed mould that will not leak under vacuum, allowing me to create strong, viable parts.

So what am I looking for here?
Confirmation that the path I am now taking is viable, or advice on how I could do things differently to achieve a fully sealed mould.

Any/all comments appreciated.

If you are planning to use the molds only once, the easiest way would certainly be to seal everything in a bag. Just wrap the backside in breather cloth und place everything on a soft blanket. If the styrene attacks the paint, there might be some defects afterwards. Maybe try out the paternity coat primer system from ec?

If you are planning to use the molds only once, the easiest way would certainly be to seal everything in a bag.

I could do this with some of the smaller parts, but for most of the parts this is not practical, given their size.

I have also built a flat base surface (4800mm x 2400mm) that all of my moulds sit on and I have thought about sealing this and vacuuming down to this, but again, this would not work for all of the parts I need to build, so I have not gone down that path (yet).