Ive been making mdf( Weyurhauser/plum creek brand only) doors for a little over 2 years now, and Im struggling to perfect the process of pre sanding, coating and final finish. I am spraying all WB primer and topcoat by ESC coatings. I rout both sides of the doors so I dont have to worry about cupping. the routed centers are then presanded with 150 grit on a dynabrade 2/3 sheet rectangular sander. At this point I have tried several different combinations of how many primer coats and sandings between coats, followed by two topcoats with a light to moderate sanding in between. My issue is that on medium to dark colors I struggle to stay under 4-5 coats of material and at least two sandings before I get what I consider to be a good finish. At this point paint buildup in the corners is also an issue. I would like to drop the number of coats to a consistent 3-4 coats, or less if someone knows something I dont... Any Ideas?? Also, switching to solvent primer is not an option for me

You can apply one or two coats of a 2K PU high solids transparent sealer only in the routed/edges. With high viscosity, without thinning, with a 1.8 or 2.0 tip.
After dried, you can sand it and continue with your WB process.

I know you said you didnt want to hear it, but 2K poly is the solution. We sand routed panels and profiles with 320, then 1 prime coat , 1 top coat, then add a 2nd topcoat just on the faces just for a slightly improved finish. 2nd topcoat would be optional depending on your finish standards.

Thanks its just that the area of california that im in doesnt allow solvent finishes. Looking for Ideas about sanding into square corners. grit process...
I think there are a couple of wb 2k primers out there I could try. Over all just trying to make fewer steps out of what seems to be a many step porcess.

Also options for 2 piece mdf where a 1/4" panel gets inserted into the frame. Drastically reduces finish time on a flat panel.

JM do you have an example of how this is done? I havent tried this because ive been trying to reduce the amount of handling and steps involved because I am a 1.5 man operation

We do it with our shaker doors. We route a pocket on the back thats just a little over 3/8inch deep (same as you would to insert glass). We then insert a 3/8 panel that is basically press fit. It eliminates warping from single side machining and is a breeze to finish. Obviously more machine time and some assembly time, but the finish is amazing, and no warping.

No experience with other methods, but here is a youtube for an option for another way to do it.

https://www.youtube.com/watch?v=dmEbKOq1mgc

I spray tons of routed mdf doors (2k solvent though). I sand the raw MDF center panel to 320 until it's almost burnished and use a high solids 2k primer, wet on wet. It does a great job filling pitting and no grain raise. I know a lot of the WB guys prefer to use a wb 2k primer because they find it helps lock down the grain. Look into Sivam, Renner, Envirolak, Icro ect, I hear good things about all their WB 2k primers. A lot of california guys are using it so it should be pretty easy to source.

Craig, what type of sander are using and what is you grit process?

Jonathan, I usually receive the doors finished sanded to 180. Most of the shops I spray for use good quality mdf for their routed doors, so they aren't terrible for pitting and fibers. I use a mirka 3x5 to sand the center panels 240/320 and almost burnish it. Then 2 coats of solvent 2k primer (Sivam) wet on wet and that locks everything down and fills the majority of the pitting. I've talked to a few guys that use wb primer and they get better results if they sand between first and second coat if primer

Are using the ESC prime master? Also the pre sanding of 150 is to rough ,
Should be done at least 220.
MDF first coat of primer should not be heavy , a light coat is best. Can either sand this to remove some of initial
Rough texture after 30 to 45 minutes blow off sanding powder . Apply 2nd coat , even wet coat , wait at least
1 hour before sanding 70 degree temp,.
Apply top coat ESC Spray master or ConVar .
You can also contact Rick or Zack at Colorama paint store in San Diego , Rick is a biochemist who is designed these coatings knowledable and will def take time to guide you through this .
Hope this helps you

Frank, yes. Using the primemaster. Im beginning to see that the 150 is too rough. In the beginning of making cnc shaker doors, my machining was a little rough, and required heavy sanding. Im not sure how familiar you are with ESC products, but over the last two years ive witnessed a chemistry change in the lineup. The topcoats, including the clears, seem to be drying much slower than previously. I was able to compare with older product that I still had in stock. I've also had issues with colorant overload on dark colors. This resulting in even longer cure times and on a couple of occasions cratering. Positively, the primemaster blocks stain better than any of the other WB primers that I use, and sands well too. It doesnt seem to isolate, or lock down the grain as well as some others that Ive used, namely chemcraft aquaprime and mlc agualente. Ive had good service from ESC and they have an extremely complete line which is very attractive for me, but product consistency has been a little off with the topcoats. Another positive is that once cured all of their topcoats are super durable. More so than many WBs that ive used