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).

As long as you have a room with enough space for this and enough soft materials, I don't see why this could be a problem. Place a bagging film on a soft surface, put tacky tape down on it, some breather cloth, the mold, you reinforcement etc, than the second layer bagging film

One of my parts is 4600mm x 2300mm x 900mm. There is literally no way I could encapsulate that size mould in a bag, as I am working alone on this build.

I appreciate the suggestion, but as I said, this is not really an option. What I do need is a method that positively seals the MDF.

2 coats is almost always the way, the first coat sealing the MDF, the second giving you a smooth surface for you to apply release agents onto.  You will get some resin soaking in, depending on the grade of MDF so relying on one coat can be unreliable if it soaks in more on one area than another.  By doing a second coat you will avoid that problem.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thanks for the comments/suggestions guys.
I would not assume that a single coat of anything would be sufficient for sealing MDF, especially the edges. There will be at least two coats, maybe three.

As mentioned initially, my greatest priority is that whatever I use must key well to the MDF and have no porosity.
I am also going to try spraying the top coat of epoxy resin, as brushing on the epoxy leaves some brush marks, even if tip brushed.

I have also thought about using gelcoat directly on the MDF, but I am not sure how, or if, that would key well.

My test moulds are build. I am just waiting until I have a bit of time to coat and test them.

Gelcoat would bond fine, but it wouldn't be the easiest thing to get a smooth surface with. I hate using gelcoat for anything other than in-mould layup.

I've done this before.  I used a brush with a 2k lacquer.  First coat seals up the porous nature of the mdf then a light sand to denib before a couple more coats.  Once it stays glossy spray a coat on and run a polisher over it.  Mould sealer and release its good to go.  Barely adds any thickness if you're carefull with it.  I've never used the bespoke products as I'm too tight but I've heard they're all pretty good.