Fiberglassing Square Edges
Occasionally one needs to bring fiberglass cloth around a sharp edge. This could occur on the trailing edge of a rudder, for example. Those who have tried this know that it is almost impossible to do. The fiberglass is just too "springy" and lifts from the edge creating air pockets. These eventually tear or fill with water. In either case the wooden substrate gets wet and the reason for having the fiberglass there in the first place is lost. While it is possible to keep pushing the fiberglass back down until the tackiness of the resin finally holds it in place there are far better ways to do this.
One way is apply a coat of epoxy on either side of the edge an hour or so before applying the tape. Then simply unroll the tape into either side of the tacky epoxy and pull it tightly around the corner. Once it is smoothed out (get it right the first time as it will be impossible to move it around much) it can be wet out with fresh epoxy. This method is tricky for several reasons. The epoxy base has to have the correct tackiness. It has to be placed right the first time. The freshly mixed resin may soften the tacky layer enough to allow the fiberglass to "jump off" the edge. Pressing the tape too far into the tacky layer may make it impossible to thoroughly wet it out and white areas occur. But, if you are doing a bright finish boat or other object this is about the only way to do it.
A far better way to do this (but one that only works on a surface that will be painted) is to remove enough of the edge so that the "roundness" starts back about a half-inch from the former edge. Replace the removed material with SilverTip EZ-Fillet, Quik-Fair or an epoxy putty of your own making. Overfill the now rounded edge enough so that it can be block sanded using the two surfaces adjacent to the former sharp edge without leaving voids. Then, once the putty has cured, sand back leaving a "square" edge right where the old edge used to be. At this point one simply applies fiberglass to one surface allowing the glass to overrun the edge by a half-inch or so. Trim this side of the fiberglass right to the sharp putty edge, clean up any drips on the other side of the edge and apply fiberglass to the other side of the edge again allowing a free half-inch overrun. Trim this when cured.
While technically the second method does not involve bending a single piece of fiberglass around the edge it is superior to the first method. The wood is far better protected. A sharp ding may well dent the epoxy at the edge but probably will not break through to the wood. Such damage is easily repaired with SilverTip putty.
10.29.04 @ 6:18pm Malcolm C. of North Vancouver, BC Canada wrote:
Another method is to lay a strip of fibreglass tape saturated with mixed epoxy along and over a sharp edge. Cover the fibreglass tape with a strip of peel-ply (wider than the fibreglass tape) and use adhesive tape to hold down the peel-ply in the dry area beyond the area wet with resin. Remove the peel ply when cured to the "green state".
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- STR Staff