Patch the fiberglass body, how to make it smooth

[HawkBoler72]

I'm in the process of fixing my Boler's fiberglass spider cracks and enhancing the door edge. I found that it is not easy to make the patched fiberglass smooth enough. Should apply more layers of resin?
The original fiberglass contains about a layer white stuff, 2 ~ 3 mm. Anyone know what is that, is it gelcoat?
Appreciate all information and suggestions.

[Borrego Dave]

Yup, that the gelcoat. It's sprayed into the mold first then the glass, resin and matting. Others will be along with info for your finish needs.

[jokra]

Your best bet is to find a fibreglass specialist in your area and ask his advice. Where you bought the resin? You have to be careful using the gelcoat, it should be coloured the same tint as the original tint used on your trailer. In my case, I used white gelcoat and tinted it carefully with some yellow to achieve the same colour as my Trillium. Depending on the height of the gelcoat you will need to sand it smooth after with very fine sandpaper and follow up with polish. All this sounds harder than it is, just be careful with the hardener and use less when in hot weather.

[Johnny M]

Get you a sanding block and start with an 80 grit and sand by hand until you get it leveled out (mooth and even with the area around the patch with no noticeable bumps at the transitiion) fill any dips and sand again, then switch to a 300 or 400 grit sand again, then a 800, then a 1200 or 1400. Clean any dust well, use a tack cloth, then prime, let dry and sand again with the 800 then the 1200 or 1400. Clean any dust well, use a tack cloth and apply top coat paint. Wet sand with 1200 or 1400 by hand, clean again and apply clear coat.

[Johnny M]

Also, I noticed the crack around your door, you probably have some flexing or movement of the body, usually indicate wood rot in the floor or the wood at the toe kick inside the door bottom under the lower bench. The rot allows the body to move at the front independent of the frame. This really needs to be addressed or that crack will most likely return. It will also cause issues with your door fit including a sagging door or one that seems to loose it shape, sometimes it is the door loosing its shape, or the body changing causing the door not to fit, or both in my case.

[jwjenn]

we used Interlux Watertite Epoxy on our 1999 Burro widebody to smooth over lumpy fiberglass, sanded with 80 grit then 150 per product instructions. had to lay on a couple of layers in some places and re-sand because we've never done this in our lives! we will prime the spots, sand the whole body, and roll on Brightside paint next. will post in a few years to tell how it held up. others have recommended Kitty Hair as a fairing agent... we're going all marine products, probably total overkill...

[HawkBoler72]

Thank you - Johnny, I have enhanced the frame, added a support behind the door frame, and hope these will fix the original problem.

Thank you all, for your suggestions, it seems that only way to achieve smooth is the repeat of the combination of fiberglass and sanding. I will spend more time to prepare it.

[Borrego Dave]

Eric, one nice thing about FG, if it isn't good enough it's easy to grind it down and try again. It's a very forgiving material to work with.

[k corbin]

You can also use System 3 Silver Tip Quick Fair putty. It is a two part mix that has micro balloons in it. It is ready to hand sand in 3 hours. There are a variety of kit sizes for large or small jobs. Not too bad on the pricing for an epoxy product, less expensive than many of the competing products for this application. The advantage of the micro balloons is they fill up the space without adding a lot of weight and it is considerably easier to sand than an epoxy filler that does not have micro balloons in it. It does leave a nice smooth finish depending of course on your ability to sand smoothly. But I do like it because you can overfill the patch just a little and not have to spend hours trying to get an overly hard filler sanded fair.