The countertop is particleboard? Or MDF?

Nothing more? Like laminate/tile?

handymack -- it's a wilsonart laminate over a particle board substrate. there's also a strip of what I think is MDF along the front edge. The only raw part is the underside, so I'm talking about urthethaning the underside of the overhang along the front, and the back edge that will be against the wall. All other edges are finished laminate. It just doesn't extend under the front as I've had on some production laminate countertops.

That strip should be trim and finished.

If the back is caulked to the wall, I have never seen any reason to finish it. There is no reason not to seal the back to the wall, BTW.

As far as the underside of the front, there is no harm in applying a finish, but I would not use poly. Any finish other than paint will be affected by the steam/water vapor and fail. If you paint, be sure to primer(Zinsser BIN is a good one).

Having ripped out many countertops in the past, some of which were 30-40 years old, I have seldom seen any of the damage you fear. Most damage was because of bad lamination or water damage from leaks.

Particle board deterioration is common under the counter at the sink and dishwasher. Just about any kitchen I go into has this issue with a flakeboard substrate. If the counter has one of those 4" backsplashes and thit wasn't caulked and sealed when installed, you will see pronounced swelling behind the faucet.
I would definitely seal the underside of any flakeboard counter.
But I got to ask, Why would you ever use this to begin with? If I get a customer who wants a laminate counter, the substrate is always exterior grade 3/4" ply. It's the only material the fabricator uses.
Ron

Laminate manufacturers like Formica and Wilsonart specify the use of partical board as a substrate for their material. It's smoother and does not telegraph grain like plywood can, especially with the thinner grades which are the ones most commonly sold in the postform laminate era.

"Does it make sense to protect it where possible with a couple or three coats of polyurethane, or something? "

Yes it does. Any enamel paint should do. Don't use hide glue, it is moisture and heat susceptible.

Thanks for all the answers.

In case anyone was wondering, I got this countertop online from a manufacturer who makes them to spec and ships them. It was well done and very well packaged.

I guess I should have considered (oil-based) paint. It's been so long since I used anything else, to me "paint" means latex.
I have used clear polyurethane for this kind of thing before.

Ron, you sure are right about those spots that always get damaged if they're not protected/installed properly.

What is an Particleboard Counter ? I only know Particle counter - but that really can't be similiar ones

Here is a link that might be useful: Particle Counter

The only part of the bottom of the counter that will be exposed to water or high humidity will be the edges of the sink opening so it's a good idea to seal those edges. Be sure to properly anchor the counter to the frame of the cabinet.

We used RedGard to seal this area; rolled it on like paint. Seems to be working very well; after a few years of hard use there's no sign of steam damage over or near the dishwasher.

Here is a link that might be useful: RedGard

I like your idea, I won't to make a cutting board out of left over counter top. I think this will work just fine. Thanks for your input.

My very-much-on-a-budget family has a nice new Wilsonart laminate countertop. I ran my finger under the overhanging edges and, sure enough, there is laminate on top and particle board underneath.

My kids splash a lot (the previous countertop disintegrated due to splashing which caused water leaking which did about what you'd expect to fiberboard), and I hate messing with anything oil based if I can avoid it plus I'm terrified of ruining the look of my beautiful new countertop by smearing something icky on it, so after much internet research, here's what I'm trying:

(1) two coats of Minwax water based polycrylic, which is alleged to give a crystal clear finish and it apparently does.

(2) after the Minwax has had a week to cure, I'm going to pick a day when the countertop is super dry (ship the kids to Siberia for a couple days maybe?) and then protect the most vulnerable area (the three feet directly in front of the sink, we don't have a dishwasher), by running a cut to fit half inch strip of duck tape along it, on the theory that duck tape fixes everything and it won't be conspicuous since it will be underneath the countertop lip. Duck tape kept the old countertop functioning a couple of years beyond its normal life span, so maybe I should just start off right and get the duck tape in place BEFORE the MDF has a chance to get wet.

____________________________

I'm quite nervous about my method.

There's a big online controversy over whether it's okay to use waterbased polycrylic on MDF - one person alleges that use of water based polycrylic will cause the MDF to separate from the laminate because hey, it's water based! - but others think it's okay, so I decided to try it. The two coats of polycrylic did cause the MDF grain to come up - it was smooth before and it's now rough to touch - but I don't know if that's good (because it absorbed the polycrylic and will now be impervious to water) or bad (because the nay-sayer who says it will cause the laminate to separate is correct.)

I'll let you know in a year or so how the countertop is getting along and whether it is in peaceful co-existence with the kids!

P.S. So I put two coats of Minwax Water-Based Polycrylic Crystal Clear on the underside of the laminate counter edge yesterday, gave it 24 hours to cure, then slapped a cut-to fit strip of duck tape along the underside.

And now I wonder why I even bothered with the polycrylic. The duck tape aligned with the edge perfectly, completely covered the exposed particle board from the front edge of the laminate overhang to the edge of the cabinet, and lapped down just a tiny invisible bit onto the cabinet. I carefully pressed it into place with my fingers to make absolutely sure there are no air gaps.

It's invisible unless you get down on your hands and knees and look up at the underside of the cabinet edge, and if you get down on your hands and knees, silver gray duck tape makes a nice match for the fancy stainless steel kitchen appliances, and as far as I can tell, the seal is just about perfect.

Do it yourselfer's motto: YOU CAN FIX ANYTHING WITH DUCK TAPE, AND IF YOU CAN'T, IT'S BECAUSE YOU AREN'T USING ENOUGH DUCK TAPE.

(:

I want to wrap it up over the lower edge of the countertop. The new dishwasher pops open and vents a lot of steam. So silver duct tape is out. Can I use some clear alternative? Everything I see is aluminum vent tape. Do they make clear duct tape these days? I know there are a million patterned ones now...my buddies kid collects them, but never seen clear.

And...we agree that duct tape will do the job in any case?

I would paint the particle board with an oil based paint or polyurethane. Two or three coats--painting about a foot past the dishwasher on both sides. Don't use a water based coating because the particle board will absorb the water and swell up. After it dries, you could cover with HVAC foil tape or whatever. Even heavy duty clear packing tape. Easy to replace when/ if it starts to come loose.

Instead of duct tape, have you considered pvc molding?