Gel Coat

Restoring a boat is always a learning experience. Sometimes the learning experience saves you time, sometimes it is more painful and doesn't. I have learned more about gel coat than I wanted to over the last several weeks. I would guess that most of the fiberglass pioneers of the 60's and 70's have felt the same way at times. Just when they started to think they had it sorted out, hull blisters became a scourge. Fortunately, Jet Stream has no blisters, but she certainly does have quality problems associated with her gel coat.

The gel coat on the topsides and hull of Jet Stream presents some very interesting challenges for the restoration project. After removing the paint from the hull and topsides, I found severe crazing in the gel coat above and below the waterline. There are several types of crazing defects. Bob Lacovara, Director of Technical Services for CFA (Composites Fabricators Association) has written an excellent article describing gel coat defects and this link will bring up his paper in MS Word format. Incidentally, this is not only the most concise and complete treatise about gel coat cracking that I found, but it is one of the very few cracking gel coat resources out there.

"Getting a Handle on Gel Coat Cracking" by Bob Lacovara, CFA Technical Director
www.cfa-hq.org/resources/documents/gelcoatcracking.doc

The following pictures show the gel coat problems on "Jet Stream":

Stancheon MountImpact Gel Coat
Stancheon StressImpact Stress, Stb. Qtr.

Waterline Gel CoatScupper Gel Coat
Boot Stripe StressDeck Scupper Crazing

Thru Hull CrazingCabin Gel Coat
Through Hull CrazingCabin Top Crazing

Gel Coat PorosityHull Stress Gel Coat
Gel Coat Porosity, Port BowUnderwater Hull Stress

Since gel coat has no reinforcement, tensile stresses generated by laminate flexing, can become cracks that extend at least to the mat layer if the gel coat hardness is not matched to the laminate stiffness at manufacture. This can also occur if the gel coat is allowed to harden too long before the laminate is applied, thus forming a weaker bond with the laminate. This could happen if the gel coat was applied to the mold on Friday and the laminate not applied until Monday. Cracking is very common around lifeline stanchions, cleats and other areas of stress. Jet Stream has some of this, although not more than I would expect. Gel coat can also crack from temperature stress, temperature cycling and combinations of thermal stress with embrittlement (a loss of tensile properties through a polymer degradation process). Evidence of this kind of crazing can be seen around the water line.

The extent of the crazing in Jet Stream's entire surface is extreme. In looking for why this has occurred, I purchased a Barcol tester (The Impressor, from Barber Colman) to test the surface hardness of the gel coat. When I used the tester, I found the surface to be between 55 and 60 Barcol over much of the gel coat surface area. This equates to about 38 to 45 Brinnell or 75 to 83 Rockwell H. Gel coats that have been properly cured but have not become embrittled, are generally in the area of 40 to 45 Barcol. Harder gel coats are sometimes used to maintain good gloss but that property must be balanced with a stiffer laminate to prevent cracking. While I may never know exactly why the gel coat has failed, it is probable that it is related to more than one cause. It is likely the combination of a very hard formulation in combination with a fairly flexible laminate structure and the possibility of poor bonding between the gel coat and laminate. I also contacted several fiberglass experts and asked a lot of questions. Several points have come out of those discussions:

a. The experts all agreed that the problem was severe.
b. The cracks appear to stop at the mat layer.
c. Whatever treatment I apply to the gel coat will probably allow the crazing to print through, over a period of time.

I have had several suggestions for a "fix" that included the following:

1. Encapsulate the entire boat in a layer of 'dynel' (a polyester cloth with more flexibility than glass cloth) and epoxy
2. Coat the entire surface with low viscosity epoxy during a hull cooling cycle to draw it into the voids
3. Just prime it with high build and don't worry about it.
4. Paint it with a light color to minimize future temperature extremes and cycling.

While all of these suggestions have some merit (at least in theory), my engineering background tells me that I would be attempting to hide the problem rather than correct it. Whenever possible, I prefer a solution that corrects the problem. The only ways I can see to correct the problem, are to either treat the gel coat to permanently soften it slightly, or remove the gel coat. Since permanently softening the gel coat seems unlikely, that leaves only the removal option. This of course will be followed by epoxy application to put back the thickness removed. I believe I can get the thickness back with about five coats of slow setting epoxy rolled and tipped. A factory rep from MAS Epoxies advised me not to try spraying the epoxy, as he believed that the rolling and tipping would produce a more accurate thickness. The gel coat to be replaced, is in the order of .020" to .031", so 5 coats of 4 mil epoxy should be adequate. This of course will be followed by primer, high build, fairing compound and eventually two-part polyurethane (above the water line).

Gel coat removal or "peeling" has become fairly common for blister repair below the water line, but is not very common above the water line. There are robotic planers for work below the water line, and a surface not quite fair under a coat of bottom paint will not be obvious. Above the water line, removing gel coat while maintaining fairness requires good tooling and procedures. I found two hand-held machines designed for removing gel coat. One is the "Marine Shaver" from AIT Tool http://www.aittool.com/marineshaver.html and the other is GelPlane from GelPlane International http://www.gelplane.co.uk/gelplane/product.asp. The biggest difference between the two is the actual contact/cutting surface. The Marine Shaver is a horizontal rotary tool that by its geometry, cuts a planar surface. The price is about $600.00 in the U.S. The GelPlane cuts with the edge of a cylinder against the surface to produce a cut capable of following a simple convex or concave curve. The GelPlane is manufactured in the UK and sells for about $2000.00 in the US. Both use carbide blades and vacuum dust extraction.

Although the GelPlane is more expensive, I have opted to use it since its cutting geometry should be far more friendly toward keeping a fair surface. The work has just begun so I will have more to report after my learning curve is more complete. I have started by removing the non-skid with the GelPlane, since any mistakes will easily be hidden with new non-skid. I do worry about all of the corners in the cockpit and the deck to cabin juncture as the GelPlane is not designed for those types of surfaces. It really is designed to peel a bottom that is primarily made up of convex curves. I have yet to see a good material removal tool for tight inside radii.

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