Can Anyone Recommend a Sealer for MDF?

11 years ago

I'm re-painting some window sills in our home. They are made of MDF. The previous owners had applied latex paint. There were deep brush marks so I had to sand quite a bit, exposing the bare MDF in some areas. I primed with Benjamin Moore Superior Fresh Start primer and then painted with Ace Cabinet and Door paint. The spots where the MDF was exposed show through the paint as slightly raised areas. Before I painted, I called our local Sherwin Williams store to ask if I should use an oil primer but they said water based would be fine and wouldn't cause swelling.

I researched some threads on painting MDF and came across one that said MDF needs to be sealed before primer is applied. Can anyone advise as to what type of sealer I should get? Thank you.

Comments (2)

Michael
11 years ago
This may be simple, or it may become very complicated.

Once the MDF is exposed (factory prime coat removed) it becomes affected by room moisture and any water based products. MDF is not suitable for window sills, but it's too late to change that.

To fix it, you'll need to feather sand it flat and smooth and apply a sealer such as GARDZ. Once the sealer has hardened (3 hours), smooth over it with a quality spackling compound such as MH Heavy Patch. Sand when dry. Apply another coat of sealer, and then paint the entire surface for a uniform finish.
kendog2
Original Author
11 years ago
Brushworks, thank you so much for your helpful reply. I have learned a lot from you on this site.

You're right that it is too late not to use MDF for the windowsills. They were installed when the house was built in 1990. However, I appreciate the advice as we plan on installing wood casing around some of our windows in the near future. At that time, we'll probably replace the windowsills as well. Would you recommend poplar?

I found that Home Depot carries Gardz only in the five gallon size. Would shellac work just as well? I haven't been able to find MH Heavy Patch. I do have some drywall texturing compound on hand. Would that be okay to use in place of the spackle paste?
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