CONTINUOUSWAVE

A 1970 13-foot hull has several cracks and holes. I have read about hull repair but am still not clear on a few items.

Based on what I read, it seems that epoxy resin over fiberglass mat is the best approach for patching holes, and I am familiar with the process. However, I was told by a reputable manufacturer to use polyester resin, primer, and bottom paint for below the waterline.

I've also read that gel coat resin doesn't adhere well to epoxy resin.

Q1; is epoxy resin the preferred resin for below-the-waterline hull repairs?

Q2: specific to below-the-waterline repairs, do you [apply] gelcoat over the resin?

Q3: Or, do you use primer and bottom paint over the resin?

In case anyone reading this thread is searching for related content, here are two good [articles hosted on CONTINUOUSWAVE.COM]:

Repairing Hull Damage the Whaler Way
http://continuouswave.com/whaler/reference/whalerRepair.html

The Epoxy Cure
http://continuouswave.com/whaler/reference/epoxyRepair.html

BoatworksToday is also a great resource.

I've also read that gel coat resin doesn't adhere well to epoxy resin.

There is an old-wives'-tale that gel coat cannot be applied over epoxy resin. This is not strictly true.

Many epoxy resin formulations will cure with a amine blush layer atop the resin. This amine blush layer must be removed before using a gel coat resin as a top coat. The amine layer will interfere with the curing process of the gel coat resin. The amine blush layer will not be removed by solvents like acetone. Amine must be removed by using water and a detergent cleaner. As in any process of applying a top coat, preparation of the surface is very important. As long as the epoxy repair area is thoroughly cleaned and washed, there should be no difficulty in applying a gel coat resin top coat due to the underlying surface being fully cured epoxy.

I recommend you listen to my interview with Jim Watson of WEST Systems on this exact topic. I spoke with Jim about twenty years ago in order to resolve many of the same questions you ask. Visit

http://continuouswave.com/whaler/radio/index.html#west

and choose the interview with Jim Watson. There are two parts. This will give you more background on the use of epoxy resins in the construction and repair of boats.

Regarding use of epoxy to make repairs: epoxy is a strong adhesive and can form secondary bonds of high strength to other surfaces including polyester resin surfaces. When a fiberglass boat is laid up using polyester resins, the layers of resins are applied before the underlying layer has cured, so the polyester resin is curing with itself, forming what is called a primary bond. When polyester resin is applied over fully cured older polyester resin, it forms a secondary bond. Because epoxy resin is a stronger adhesive, it forms a stronger secondary bond to the cured polyester resin than a new layer of polyester resin can form to old and fully-cured polyester resin.

Epoxy resin is also more water resistant than polyester resin. Boston Whaler boats were molded with polyester resins, and require use of a protective two-part sealing layer (usually an epoxy paint) in order to create a water-resistant layer that will be immersed continually in seawater. This sealing layer is not the anti-fouling paint usually seen. Anti-fouling paint discourages marine growth. For advice directly from Boston Whaler, read the owner's manual. If you do not have an owner's manual, you can read my HTML version that I created from the original printed manuals. See

http://continuouswave.com/whaler/reference/manual9-17/

Read the section on bottom paint at

http://continuouswave.com/whaler/refere ... tml#bottom

Regarding the choice of using epoxy resin or polyester resin to make a repair to a Boston Whaler boat hull, I can offer some additional comments.

Generally a newly-applied repair made with epoxy resin will cure to much harder surface than the much older--perhaps 50-year-old--polyester resin of the original surrounding areas. For this reason, when fairing the repair area into the hull surface, one must be careful not to removed too much of the original and now soft polyester resin at the joining lines.

Many repairs to boats originally made with polyester resin are done with polyester resin, and the results are generally good, as long as the polyester resin is able to cure properly. Generally laminating polyester resin will not cure to a hard surface, as it expects to be top coated with a polyester gel coat resin, to which it can form a new primary bond.

Gel coat resins can cure to a hard surface, but they must not be exposed to air for this to occur. Typically a gel coat resin will be top-coated with a layer of poly-vinyl alcohol to prevent exposure to air, or the resin will have an additive (such as a wax) that will rise to the surface and create an layer to prevent air from reaching the gel coat.

Polyester resins are essentially already in the process of curing while in the air-tight can, and the curing time is greatly accelerated by adding a catalyst, usually methyl-ethyl-ketone-peroxide (MEKP). The mixing ratios are very small, about one- to two-percent by volume. For this reason it can be somewhat difficult to precisely measure the proportions of resin and catalyst when making a small batch.

In the end, I believe the choice between epoxy and polyester resin for repairs is somewhat akin to deciding if you like Scotch Whisky or Irish Whiskey. Either can do the job.

From my experience in making small area repairs, I found the convenience of the WEST Marine repair kits in which the epoxy resin and hardener components are pre-measured at the proper ratio and supplied in small foil packets to be much simpler to work with than brewing up a batch of polyester resin, guessing at the amount of catalyst, and getting a hard cure.

An additional article regarding repair procedures for a Boston Whaler boat is from Boston Whaler themselves, and is reproduced in HTML at

Instructions: Hull Patch Kit
http://continuouswave.com/whaler/reference/repairInstructions.html

These instructions use "general" polyester resins or gel coat resins for repairs and date from 1994.

Thank you for all the info. I've printed and read everything but would still appreciate clarification on the products to re-finish the hull. From what I've read, the sequence for finishing the hull is as follows:

1. Apply gel-coat to entire hull.
2. Apply two part epoxy paint over gelcoat but only below the waterline.
3. Apply anti-fouling paint over the two-part epoxy paint but only below the waterline.

Is that correct?

OP: It appears you may have items 1 and 2 in reverse