The compound is porous and has absorbed more of the paint than the other areas. People prime first to even the field before painting but newer premium paints work better to eliminate that step to avoid what's called flashing. Always ask, research or test under various lighting conditions first as it depends. Probably safer and easier to just prime. (I did try not priming a good sized patch, using BM Aura Matte paint, and cannot see it. I simply applied another coat of the Aura over, but it might take two in some cases. Holds up to grazing with a light but this area does not see the light of day! I was surprised but it worked well on a wall that already had two coats of fresh paint. Why did you use drywall compound on seams and nail holes? Caulk is used for seams (get a high quality flex elastomeric like Dynaflex 230 or BM Crown & Trim to reduce the potential for cracking). Wood filler is used for nail holes. There are some differences to working with MDF rather than wood but I don't know what they are.

MOST of the time... We usually tell customers that "pre-primed" = "RE-PRIME"!! Pre-primed goods are usually pretty cheap coatings. After a month, they're (the primers...) are too dried-out and brittle to bond very well anymore. To clarify though: Do ya mean the TEXTURE of the knot is showing; OR, is the knot BLEEDING something thru to the topcoats? * You could also just use the BIN spray (if possible), or brush this on. * Sanding would help level things out more though...but it would be a pain cuz it's installed! Faron

Well, if it really is a stain, you need to use original oil based Kilz or some other oil based stain killing primer or BIN. Kilz 2, the water based version, is really not capable of killing any stains that I have seen. They may as well remove the words 'stain blocking primer' from the can entirely because it is nothing short of a big fat lie.

What paint/primer brand and type should I try? I see on Consumer Reports the Behr (Home Depot) Premium Plus Enamel is rated tops and great for for hiding (whatever hiding is). In the last house we had a similar issue with Behr semi gloss paint and the Kilz primer. Not sure if it was the same type of paint but I painted 2 coats of Kilz primer and several coats of semi-gloss white but it was not covering. I gave up after about 5 coats total. The rest of the house had semi gloss painted by the home manufacturer and it looked very nice and cleaned up great (kids). The one room I painted had the flat paint and the texture would come off the walls when you had to clean it. Current house has similar flat paint everywhere and we hate it but can't seem to get the coverage the home builder did in the last house with semi-gloss paints. we're about ready to give up, costs to much time and money to put on 5 coats and still not look good. I'm going to wait at leat a week or more before trying a new primer and paint. Wish me luck!