I have laminate countertop that I was going to use outside and thought I'd ask what the best sealer for the MDF to use, Flex Seal, Polyester Resin, etc.? Thanks.

 

I assume you mean to coat the underside - a coat or two of most any solvent based coating will seal the MDF,

 

But if it gets even a little moisture it will swell like crazy!

For anything outdoors you should consider something like stone, granite, cement, solid laminate. Anything that is not going to react to water!

 

Do everything you can but backer material used in the pre-covered counters found in most home centers is not the greatest. If the counters with built-up the MDF used underneath might be slightly better but still is likely very similar. It loves to swell when it gets wet. No matter how much you do to seal it I think it will eventually swell. All you can do is try to delay it as much as possible.

I have treated folding tables to withstand occasional "weather" and have had good luck doing a first coat with oil based polyurethane or spar varnish diluted about 50/50 with mineral spirits so it soaks in more deeply. After the first coat is dry I follow up with more diluted poly if there was any left otherwise I use full strength for the second coat. Just keep sealing until you're tired of sealing. You want to seal up all the pores and pin holes. I tend to think the more coats the better.

About a year ago I made some tables that spend their life outside. The tops are Advantech, a high quality oriented strand board. They are under cover but still get a lot of rain blown in on them. I applied a couple heavy coats of Zinsser oil based primer. The oldest table is a year old and still holding up well.

 

Solid laminate?

Can you explain? Everything with which I'm familiar would say this is a contradiction in terms.

 

It's laminated on the top of MDF. It's an old kitchen countertop. Thanks.

 

Yes. To undercoat the underside. Thanks for the advice.

 

Definitely coat the bottom side in addition to the edges. Condensation and water wicking/sticking on the sides can get underneath so it needs to be protected as well. Any seams and the exposed MDF on the sides will need the most attention. If you have seams in the top I might even go as far as to route a small groove along the seam so you can put in a good bead of silicone caulk.