Plexiglass window restoration

These are instructions for installing new windows without fasteners on the L28. The boat flexes and the original window fasteners cause the windows to crack and leak after very little use. I replaced my windows 3 times over the years using the original fasteners and they leaked like crazy every time no matter what sealant was used. Special sealant now bonds the window to the gelcoat and it works perfectly without any fasteners. But if you use fasteners the windows will crack and leak... so fasteners are a no no. Learned this one the hard way. There are a few tricks below that describe how to do it properly. 

Buy a new set of Acrylic windows (plexiglass not Lexan) that are 3/8" bigger than the perimeter of the original windows all the way around. This gives you added gluing surface that will be necessary in the absence of fasteners.

You need to clean the gelcoat fiberglass surface very well by scraping all the old sealant off and using Lacquer thinner on a rag and on a scotchbrite pad to be sure you have a super clean surface on the gelcoat. This is very important to have it perfectly clean without any remnants of old sealant left behind because we are only using sealant to bond the new windows to the gelcoat.

Make some small wood blocks. It's simple and they are used to align the new window to sit in to the correct position.  Make 3 per side and put some of the tape (3/16" thick foam tape with glue on each side) on one side. Then I used the old window and put it in place with 2 fasteners (one at each end) then I used another small piece of wood that is 3/8" wide(because new windows are bigger by 3/8" all way around) and rested the small piece of 3/8" wide wood against the bottom edge of the old window and on the ends in all 3 places and then stuck those 3 pieces of wood in place on the gelcoat. Now you remove the old window and put the new one in place and you'll see the cut out in the deck for the window is perfectly centered in the new window.

Using the same foam tape cover the faster holes in the deck all the way around. We stick it on so that it just covers the fastener holes and then we trimmed it flush with the edge of the fiberglass hole with an xacto blade. Now all you do is peel off the film on the outside of the stuck on tape and lay the new window in place against the wood blocks and push it in place firmly so that the window sticks to the tape well. Then you can remove the wood blocks and start with the caulking Dow Corning 795.

The trick now is to get the sealant to fill the gap that is left in the oversized window. So just using a caulking gun doesn't get the sealant all the way in against the tape. We had to use a couple of 3" putty knifes (two of us working at it) to pack the sealant into the gap so that it goes all the way to the tape sticking the window. This takes time and is messy and will require a major cleanup with solvent after it's done. Plexiglass can only have varsol or mineral spirits used on it. But for the fiberglass you can use Lacquer thinner because it so much stronger and faster. You can touch the edges of the windows with Lacquer thinner but not the flat surfaces. This is the hard part because it's easy to spill lacquer thinner on the window surface. Lacquer thinner will mark the surface of the see thru areas of the plexi if that happens. If it happens and marks the viewing surface it can be polished out with compound. As you will learn Lacquer thinner cleans so much better than varsol. You tend to use very wet rags with lots of solvent on them.

You will need 2 large tubes of Dow Corning 795, 3 small blocks of wood, plus 3 x 3/8" wide woods shims per side, you can use the same blocks on the other side afterwards.  Lots of paper towel and rags and try not to do this in the sun or the sealant will skin too quickly. We did it with the cover still on the boat. Goodluck!