If you’re getting water inside, Jchannel is probably the least of your concerns. Something isn’t sealed correctly. Unfortunately, if you have water getting in, you have water in your wall. Even if you seal the window, you could end up with a mold issue.

If it was me, I’d pull the window, ensure all flashing and housewrap is in place, the reseat the window in a thick bead of silicone. Assuming the window is still in good shape. Otherwise, a new window is needed. Once the window is in place, trim, jchannel, and siding would be put back.

Once the window is properly installed, I’d focus on the inside. Pull the trim and drywall around the window, letting it air out for a day then coat the inside sheeting with mold killing primer. New insulation would be added before new drywall is put in. Then mud, sand, paint, and reinstall the trim.

It’s a decent amount of work, but not insurmountable. My concern is, if it was installed incorrectly, chances are, your other windows were too. I’m not trying to be doom and gloom here, just trying to forewarn you. It’s something to keep an eye on.

All of this is assuming the problem isn’t something like a window left open or something like that.

Good luck to you. Post the results when you finish. I’m sure more than just me would like to see it

Shellac is not the same as oil based. Oil based is made from, well, oil. Shelllac is made by crushing the shells of the lac bug. It is an insect that is cultivated specifically for the purpose. Shellac is typically used as a final coat for protection. Shellac primer is basically that mixed with a white pigment. It is great for sealing. Fairly pricy though.

I will echo the Bondo route that others suggested. The big box hardware stores sell quart cans of general purpose Bondo in the paint aisle.

Cut out the vaneer over the bad area with a razor blade or box cutter.

Sand with 80 or 120 grit to remove any loose bits of the board. The particle board will crumble a little, but it will be fine. Just dont go overboard. Vacuum it to remove most dust.

Using a flexible putty knife, get one to two globs of Bondo on a clean piece of cardboard. Don’t do too much as you will have about 5 minutes of working time once the hardener is added. Follow the instructions on how much hardener to add, you wont need much. Fold the hardener into the Bondo with the putty knife until it is a uniform color.

Spread the bondo over the area you cut out. Try to get it smooth, but dont overwork it. It doesn’t have to be perfect.

Using 60 or 80 grit paper, sand it as it starts to dry. If it is gumming up the paper, it is still too wet to sand, give it another minute or two and try again. You are shaping it with this step. If you let it completely dry, it is hard as a rock and more difficult to sand.

Once it is in a good shape, switch to the next step up for sanding and repeat (60 to 80 to 120 to 180 to 220). Do not skip gits or you will just make work for yourself. You can probably stop at 180, but since it is next to vaneer, i’d go to 220. Again, if it is gumming the paper, wait a few minutes. Once you are at 220, use it to sand the entire cabinet to rough up the surface and accept primer better.

Once sanded, wipe the entire cabinet down with a tac cloth. I also recommend wiping the cabinets with a wax and greese remover so everything will stick better.

Now it is time for primer and paint. For a smooth finish, use a good quality foam mini-roller and foam brush for tighter areas.

Prime the cabinet using any water-based primer; killz is pretty good. They make an oil based that would be better, but will smell up your whole house, so I don’t recommend it unless you have excellent ventilation.

Lightly sand the peimer once it is fully dried with 220. If any spot is too light, do a second coat. Wipe up the dust.

Paint with an alkyd paint. Do at least 2 coats.

Not necessary, but if you want extra protection, you could use a foam brush (not a roller here) and do 3 to 4 coats of a water-based polyurethane on top of the paint. It might be overkill for what you want though.

This might seem like a lot, but you will have a great finish when done. Also, fair warning, Bondo has a strong smell. Open a window.

My misunderstanding. I thought you meant from the heater to the floor.

To fix the gap you were talking about, you could do some acrylic caulking. You could do high heat silicone if you were worried about the heat, but the acrylic should be fine. Check the tube for the allowed heat range. The silicone will do vetter against the expansion and contraction, but you’d never be able to paint it. The acrylic would be paintable. But had a high chance of cracking.

You need the gap. Baseboard heaters work by heating the air around it, causing it to rise. This rising air is replaced by the cool air being displaced. By having a gap under the heater, the cool air has a place to go and be heated. This causes a draft that allows the full room to be heated, not just the area around the baseboards. In essence, the gap allows the heater to be more efficient.

If you insist on covering it, use a sheet of decorative metal that is at least partially open to allow airflow through it. Even that will reduce efficiency, but it wont be as bad as blocking it.