Hy guys,

I have plug made from MDF that was CNC routed.

I've read that it's really hard to achieve gloss surface if MDF is not impregnated and "sealed" properly. It's hard for me to find products that are used for example in America and I have to take DIY approach. I figured that it might be possible to use polyurethane 2K varnish but diluted 20-30% so MDF can soak it easily.
Does anyone have similar experience? 

Also, I saw some DIY ovens made completely from support frame and MDF panels with rock wool between outer and inner panels. My plan is to build wooden frame, put 12 - 16mm MDF for outer shell, place rock wool around box from inside and than make smaller box that fits in from steel sheeting of aprox. 1-1.5mm thickness. I would make it so it opens from top side like a lid. I need curing temperatures of around 110-120°C (aprox. 250°F). Inner dimensions would be 160x70x50cm and I was thinking about 3 to 4 heaters of 1kW strength. 

Will this kind of build be ok to withstand those curing temperatures because I am afraid of oven catching fire? 

Also, I am looking for good PID controler with ramp and soak function so I can make proper curing cycle for pre-pregs. I've found some OMEGA controllers, but I am not quite sure what's good and what's not so please if you have suggestions feel free to write.

Dentex, I recommended to you, don't experiment.
I have made a several MDF CNC machined plugs for autoclave prepreg moulds. There are a coulple ways how to do that efectively. I recomended to you, use only epoxy based coats - post cured, certainly not PUs. If the mould/plug is a single-purpose and hand laminate only, 2K-PS filler is ok.
If the plug/mould is a MDF massive block, the tested temperature resistance is 65C, for tooling prepregs enought. Alternatively, instead the tooling prepregs, is possible to use dry carbon cloth + infusion. In both cases, you have to post-cure the mould - out of plug. Very slowly going up to working temperature + 10C min. Remember that the post-curing process can ruin the mould because distorsion and cracks. Depends on mould shape the steel/AL support frame can help, especially for flat large moulds.
I have tested the MDF sheet  18mm thick and on 60C starts the distorsion, for massive block 150mm thickness on 80C seems a quite stable.
If the CNC available, start thinking about  epoxy tooling boards or AL machined moulds. That's the best, high accuracy, high effective way for prepreg moulds.

The oven heater plates or spirals are too massive pieces of steel. It means, has high thermal inertia. It means if the controller switches off on target temperature, in real the temperature goes up uncontrolled by tens of C. The heat is acumulated in the plate. That's the real problem for ON/OFF controller, not so for PID if is right tuned. Better to use the heat resistant wires, for example Kanthal based.
Good luck