Déjà vu anyone?
In brief, with MDF, my surface finishing process goes something like this:
1. Fill holes, cracks etc. with Bondo
2. Dry sand the surface with 180 and then 320 grit sandpaper. Until almost all surface trace of Bondo is gone and MDF is smooth.
3. Spray 1st coat with High Build Primer/Spray Putty.
4. Dry Sand with 320 grit for a uniform smooth surface. Wipe over with damp cloth.
5. Spray 2nd coat with High Build Primer/Spray Putty.
6. Dry Sand with 600 grit. Wipe over with damp cloth.
7. Spray 3rd coat with High Build Primer/Spray Putty.
8. Wet sand with 600 grit. Wipe over with dry cloth.
9. Spray 1st (and usually only) coat of Primer – this is less viscous than high build primer for the fine details.
a. Repeat above step 9. if required.
10. Wet sand (lightly) with 600 grit. Wipe over with dry cloth.
Surface is now ‘paint ready’ as shown in my project.
Pictured are the automotive finishing products I use. You should find very similar at automotive specialists in your area. Unlikely to find anything like these products at a general hardware/paints store.
Standard primer:
My paints of choice and clean up solvent.
These are all acrylic lacquer based products with thinners mixed to reduce their viscosity. The high build primer products can be purchased in either propellant based (rattle cans) or in bulk tins to be used with thinners and a spray gun. I will be switching over to the latter method in the near future.
Now that I've typed all that out, I'll copy it into my "How to paint your cab the OND way." thread in the Woodworking section. For a look at how I do all the above, check out the video in that post. It's not riveting watching, unlike say, a super hyped up YouTuber reviewing the latest iPhone. But, it does have the info you're looking for.