I hear ya. I am in a very similar boat myself.
What did you use as a mould sealer?

Hanaldo (13/06/2017)

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Gelcoat would bond fine, but it wouldn't be the easiest thing to get a smooth surface with.

I agree, and it is a real pain to sand too, but as I have mentioned, I am looking at all possible options at this point.
If I can find something that works well, and does not cost a fortune, that is what I will be using.

I am in Australia, but I do know where I can source Duartec products and have thought about using this, if all else fails.

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.

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.

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.

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

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.

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

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).

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?