How can I be sure I have 15 ~ 20 mil of Gel coat in a repair?

I have been thinking about learning how to do my own fiberglass/gel coat repair for some time now. After reading all the how-to guides I can find, almost all of them mentioned the gel coat should be around 15 ~ 20 mil thick, on top of the fiberglass substrate. In the scenario of repairing a gouge penetrating gel coat and the fiberglass substrate, fiberglass material would first need to apply in order to build up the hull material. What is the industry accepted practice to make sure the fiberglass is only build up to exactly 15 ~ 20 mil below the surrounding gel coat level?

I would imagine if the fiberglass is built up to the same level as the surround gel coat, when the new gel coat is applied and wet sanded down to match existing gel coat level, fiberglass would probably show through and ruin the repair job.

Am I over thinking it? Or am I missing some important piece of information in my thought process?

Re: How can I be sure I have 15 ~ 20 mil of Gel coat in a repair?

Your glass repair is sanded down below the existing level of the glass and then filled and sanded smooth and left at the height of the original glass which will be 20 mils below the height of the finished gelcoat. When you re-gelcoat you will build up coats of gel coat to a height greater than the existing gel and then sand buff and polish down to match the existing gelcoat. That's a real basic description of the process.

Re: How can I be sure I have 15 ~ 20 mil of Gel coat in a repair?

asm_ said:

What is the industry accepted practice to make sure the fiberglass is only build up to exactly 15 ~ 20 mil below the surrounding gel coat level?

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There is none for repairs.

If the repair is small then you just kinda lower the substrate surface down enough to be able to spray your gel and sand/buff it without going through.

If its a larger repair .. then you spray beyond the repair point to give you enough 'blend' room to not leave a bump around the repair.

asm_ said:

I would imagine if the fiberglass is built up to the same level as the surround gel coat, when the new gel coat is applied and wet sanded down to match existing gel coat level, fiberglass would probably show through and ruin the repair job.

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Yup.. you will burn through the repair and have to resand and spray/sand/buff again. Make sure you prep out far enough from your repair to NOT burn through your Edges ( Your main spray will not burn through unless your longboarding your whole spray repair ).

asm_ said:

Am I over thinking it? Or am I missing some important piece of information in my thought process?

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Actually its a Very good point to bring up. Your not over thinking it.. your thinking just right .. Your thinking 'ON PLANE' .. Top down instead of bottom up ..

For larger repairs its almost impossible to keep your 15-20 Mils consistent before the spray. You just make it what it is and make it look good. its all you can do.

YD.