The Rot Doctor


Subject: Replacing stringers with engine in place.
Date: Sun, 18 Jun 2000

Hello Again

You may recall my previous email, with the same subject header, about flotation and replacing stringers. Since starting the removal of the old stringers I found another problem which I believe you may have suspected. I found a soft spot in the transom and after some digging and wet wood removal discovered that about 2/3 of the transom was previously replaced. (Why 2/3 is beyond me) After ripping out all of the bad wood I also found that there is about a 1/4" gap between the wood that was replaced and the exterior fiberglass. I drilled several holes around the transom and didn't find anymore soft spots. The wood appears to be cedar plywood, bright red and very hard. It is now airing out. I have a plan and a few more questions. The final treatment of course depends on the good doctors' approval.

The Plan:

The first thing I intend to do is treat the good transom wood with CPES (which I have ordered and am impatiently awaiting). I am going to cut 2 layers of treated plywood to fit as closely as possible the area I removed and treat that with CPES. After 2 days I will apply L&L epoxy (which I have not ordered yet) to the fiberglass and surrounding wood and put the plywood in place. Cover it all with L&L. Next drill 3/8" holes all the way through to the fiberglass and start filling with L&L and sawdust. Hopefully this will fill the gap I spoke of earlier.

This sounds just fine. When you get to the sawdust and L&L Resin, get the coarsest sawdust you can and then keep it flowable. The sawdust is mostly to take up space that would normally be taken by the L&L Resin. The L&L is a superior product but it ain't cheap.

For the stringers I am going to use a 2x4 a 2x8 and a 2x6 treated and treated with CPES and epoxied together with West (I have it I have to use it). This strange assortment of sizes approximates the original and fits the contour of the boat. At this point I am not sure of what I am doing, not that I am sure of any of this but I have studied your pages. I think I will use the L&L where the stringer butts up against the transom and under the stringer glueing it to the bottom. Use L&L to glue the front of the stringers to the support. Use cloth and L&L or West to cover it. Paint the whole mess and go fishing.

Yeah, especially the "go fishing". This sounds fine. The West will do nicely for the bonding.


1. The trim tab screws are in this bad wood. When I replace them should I drill first? Should I use 3M 5200 after the CPES treatment?

I would do this: Pull the old screws, dry the holes with a hair-dryer, apply the CPES, wait for about an hour and then swab in a little of the L&L Resin, wait 24 hours, bed everything in 5200 and re-apply screws. I don't think you'll have to re-drill for the screws, and the wood will be protected.

As a direct answer, 5200 can be used on CPES-treated surfaces after about 15 minutes or longer. If between 15 and about 60 minutes a kind of "double-bonding" will occur and the 5200 will stick even better.

2. Should holes be drilled in the stringer pieces to allow the epoxy to get a better bond?

If the stringer pieces are new wood then no holes need be drilled. Pay particular attention to putting the CPES on the stringer places where there is end-grain and let the wood absorb as much as it will. Beyond that, the L&L Resin can just be applied to the stringer surfaces for good bonding.

3. The engine mounts bolted directly to the wood before I removed them. Is this acceptable practice? Should I use the 3M 5200 on these bolts?

Yes, it's a standard practice. I would treat the bolt holes with CPES first, wait a few hours, treat again with L&L Resin, and then mount the bolts. You can use the 5200 if you wish, but you may play hell getting the bolts back out again if you need to.

You could also sand clean the bottom surfaces of the engine mounts and then apply a layer of L&L Resin to the metal and the wood surfaces before re-setting the mounts. Kind of guarantee that they'll stay there forever.

I guess what I'm trying to say here is that you can mount screws and bolts into cured epoxy without difficulty, but if you put them in un-cured epoxy or 5200 you may have trouble getting them back out. Sometimes this doesn't matter, sometimes it does.

4. The boat is in 2 fiberglass pieces, top and bottom riveted together. The rivets are getting a little loose can I epoxy these 2 pieces together to avoid removing the rub rail? Or drill right through the rub rail and use screws and bolts? Or both?

You could do both and it would be perfect, but it would probably be plenty strong if you just knocked out the loose rivets and replaced with stainless bolts and lock-nuts. I guess it's kind of an issue on how much money you want to spend on L&L Resin.

5. In the evening when the sun is just right I can see light spots on the bottom of the hull (inside the boat). Is this just gel coat knocked off or something more serious? Should I epoxy over these on the inside?

What you are seeing I think is just a variation in the thickness of the hull laminate. It's fairly common. I wouldn't worry about it unless you notice cracking on the inside or outside. If you do, then get back to me and we can talk about a fix. Unless that happens, I don't see any point in spending the money.

6. some dummy that tried to reinforce the stringers ran a lag bolt through the stringer into the bottom of the boat and pierced the fiberglass. I don't think it went all the way through but it cracked the gel coat on the bottom. How do I repair this?

I'd clean out the cracked portion as best you can, wipe it down with Xylene, apply about 3 dabbed-on coatings of CPES (wait about an hour between coatings), then another dabbed-on coating of the L&L Resin. This should fill and waterproof the cracks. If you wanted to be a perfectionist, you'd then go back over it with something like out Epoxy Filler to smooth things out. You can sand the filler smooth after 24 hours.

7. Will the CPES flow through the good cedar plywood?

It will flow through where it has good access, such as on the end-grain. On the flat surface it will penetrate about a mm or two. In either case, it sets up a semi-waterproof barrier that makes it hard for the wood to absorb enough moisture to support biological activity. And of course, as far as we know, fungi and bacteria don't like to eat epoxy.

8. Last question. How much L&L and sawdust do you think this project will require. The gap behind the transom is about 6' x 3' with 1/8" to 1/4" gap. Also how much CPES, I have ordered 2 - 2 quarts but I have the floor and supports to treat.

My best guess would be that you should get another 2-quart unit of the CPES and probably two of the 2-quart units of the L&L Resin. Both of these products have long shelf-life if unmixed...if that's of any help.

Mix the sawdust carefully with the pre-mixed resin. At first it goes in easily and quickly, but it gets thicker fast. You do want it to stay flowable enough the move down. The L&L Resin is very slow-setting, so you'll have a good hour to work with it before you have to pour -- unless the temperatures are up in the 90's, in which case you'll have to work faster.

As you can see I have quite a project for a first boat. I can't tell you what a confidence booster your input has been. Thank you and let me know if I am asking too many questions.

Woody F.


You cannot ask too many questions. We are pleased that you have chosen to use our products and our techniques to repair your boat, and we feel obligated to you as a customer to give you every answer we possibly can. Stay in touch. Let us know if we can be of any help.