View Full Version : OT - Fiberglassing

09-17-2005, 03:18 PM
I'm in the process of patching up my lawn tractor hood (plastic). I need to sandwich on a piece of plywood (or maybe sheet metal) to one corner where the hinge swivel is. But it's not really a flat surface, so I'll have to build up a thickness of about 1/4" in some spots.

My question is, would it be better to build it up w/multiple layers, or just try to lay down one thick layer? ..and if building up multiple layers, is it best to let each layer cure and then sand, or just immediately slap on the next layer?
With the temp around 90F, the stuff cures pretty fast.

09-17-2005, 03:29 PM
If it were me. I would biuld it up and use epoxy resin not polyester resin. Make sure you bolt thru and use a larger insert than you would think to use. It helps a lot to drill holes so that you get thru bonding. Epoxy will bond to just about anything but metals have to be clean, oxide free and scuffed to about 60 grit so as to provide a good substrate. Jim

09-17-2005, 04:55 PM
from what i know about f/glassing,you wait until the resin has just lost it's "tack",and then put on the next layer.It depends to some extent on the brand and/or type of resin,but usually this is 10 to 15 minutes.with polyester this could be overnight,so for speed(and less smell!),i'd also go with epoxy.BTW,old credit cards make great epoxy spreaders.good luck.


09-17-2005, 06:18 PM
A old caster off a bed with a handle tacked on it a excellent roller to mash the matt into the resin. I saw a Discovery channel special where they vacuumed and fed the resin into the double hull mold and it filled every matt cavity making a super strong hull.

fixin cracks?
I put a brace in when I do this, a wire or tig rods then glass over them, Not all resins will bond to metal thou.

Last Old Dog
09-17-2005, 09:48 PM
<font face="Verdana, Arial" size="2">Originally posted by ibewgypsie:
I saw a Discovery channel special where they vacuumed and fed the resin into the double hull mold and it filled every matt cavity making a super strong hull.

Seeman Composites Resin Infusion Molding Process (SCRIMP) is a vacuum-assisted resin transfer molding process that produces parts excellent for marine, aerospace, transportation, and infrastructure applications. My interest is an ultra light weight strong hull for our racing yacht. Last Old Dog
Not affiliaated with the authentic "Your Old Dog"

09-17-2005, 10:25 PM
What I've been using is polyester resin (Bondo) that I gotten from autoparts stores. Would've preferred epoxy but haven't found any locally. Checked with a Dupont auto paint store and while they had some higher price stuff, it too was polyester. Different brandname, but the fine print showed it also was a Bondo product.

09-17-2005, 11:45 PM
I'd lay up multiple layers, as quick as you can. The previous layer needs to be at least tacky when you lay a new one down.

Use the autobody bondo. I made a pretty nice replacement fairing for a snowmobile with it. Redneck engineering at its finest! http://bbs.homeshopmachinist.net//smile.gif

[This message has been edited by CompositeEngr (edited 09-18-2005).]

09-18-2005, 06:58 AM
Try a marine supplies store. They should have patching outfits for boats at the right price.

09-18-2005, 10:53 AM
You can order epoxy from West Marine

09-18-2005, 11:11 AM
Sorry Lynn I should have put in where to get small quantities of needed materials. Most hobby shops that build rc planes and boats will have small quantities of epoxy resin and cloth of various weights. For your application I would think that somewhere around 7oz cloth not mat or roving as these don't have the omnidirectional strength you need. Though you could use cheaper mat for building thickness between the cloth layers. By the way most of the hobby shops around here also stock carbon fibre cloth and layups

09-18-2005, 11:57 AM
polyster resin seems to have difficulty bonding with some plastics..might be worth testing a piece before going "whole hog" into it. Also, if there's any cracks (stress/vibration type) in the damaged area would probably be worth drilling the ends of these with a small hole to keep them from radiating...I'd also go for the multiple layup here...take care & good luck

09-18-2005, 12:10 PM
the fiber glass & epoxy resin kits you get for boat repair realy work good , I used 2 $20 patch kits to fix some good size holes in an old boat.

It was realy fun , it was about 100F that day so I was drinking lots of beer, then the fumes from the resin got to me and I was blasted stoned.

anyway the repair turned out great.

09-18-2005, 12:15 PM
Yeah, I take 'Wooden Boat' magazine (great magazine BTW), and it has numerous sources' ads for this sort of stuff.
But when I finally decided to jump in and tackle the project, I knew if I waited for orders to arrive my enthusiasm would die. http://bbs.homeshopmachinist.net//biggrin.gif
I should've tackled it long ago. By waiting it permitted the weakened structure to crack and break in more places. ...the classic "Stitch in time saves nine" effect!

Oh BTW, a brief comment on Bondo's business practices. One kit I bought at Autozone listed the contents: 8 fl Oz of Resin, 1 tube (6ml)of hardener, and 8 Sq Ft of fiberglass cloth, and 1 plastic spreader. The box container was shrink-wrapped in plastic. When I got it home and opened it, inside the box I found a little slip that read: "Attention A printing error has contributed to a misstatement of kit components. ...kit contains 3 sq ft of fiberglass mat, not 8 sq ft as listed on the box. Thank you for your continued patronage."

Now if they were really interested in correcting the misprint, it would have been a simple matter to put the correction on the outside (underneath the shrinkwrap) where it could've been seen. Hmmm. Can you spell fraud?

09-18-2005, 12:33 PM
<font face="Verdana, Arial" size="2">Originally posted by chkz:
polyster resin seems to have difficulty bonding with some plastics..might be worth testing a piece before going "whole hog" into it. Also, if there's any cracks (stress/vibration type) in the damaged area would probably be worth drilling the ends of these with a small hole to keep them from radiating...I'd also go for the multiple layup here...take care & good luck

I've already patched several areas, and it sure seems to have bonded nicely. I've been coarse sanding the plastic beforehand, but didn't think about drilling any stopper holes. Shoulda done that I guess. However I did extend the fiberglass well beyond the end of the cracks.

My biggest problem is in building up the areas with small chunks missing, that involve curves and such, and places requiring butt joins.

09-18-2005, 04:05 PM
For a build-up that needs strength, I use "flox," which is chopped up cotten that's mixed with epoxy to make a putty. It's strong but heavy and a little lumpy.

www.aircraftspruce.com/catalog/cmpages/flockedcotton.php (http://www.aircraftspruce.com/catalog/cmpages/flockedcotton.php)

For a build-up that's just for looks, you can mix "micro" into the epoxy (also sold by aircraft spruce). It makes a beautiful smooth filler, but doesn't have the strength of flox.


09-19-2005, 06:17 PM
I've done a bit of work like you propose to do. A few pointers follow:

1) Use Fiberglass Mat in areas that have multiple contours. Cloth and Woven Roven don't like to bend in all directions like Mat does. Mat is also strong in all directions because the fibers run in a random pattern. You just need to lay up more layers because it doesn't hold as much resin and will be thin. Resin alone isn't very strong. Much like concrete the aggregate (rock or gravel for concrete, Mat, Cloth or Roven for resins) dramatically increase strength. Mat, Cloth and Woven Roven are available in various weights.
2) If you have done previous repairs using polyester resin you can use polyester or epoxy resin to make additional repairs. It's an interesting phenomenon, but Epoxy will adhere just fine to polyester, but polyester won't adhere well to epoxy. Personally, I prefer epoxy for repairs anyway. Polyester doesn't work well with Kevlar fibers either, epoxy does.
3) Polyester can contain waxes that will come to the surface after it cures. If you wait until it cures, be sure you sand the surface before applying additional layers. The work I've done has always been to apply additional layers just as the previous layer gets hard enough to support a new layer without sagging (tacky).
4) Be certain to get all the air bubbles out as they are weak spots. Special rollers are available for this but a cheap chip brush works well too. Sometimes cutting the brush hairs shorter to make it stiffer can help with this.
5) Polyester doesn't really adhere all that well to wood. Epoxy does.
6) Remember, the wood isn't really a structural component, it's more of a filler or form in most instances. Today in boats foam core is used in many instances in place of wood because it won't rot is water enters through a hole made for a fastener etc. In some cases Stringers are hollow fiberglass castings that are later laid up to the hull. Consider using cardboard as a form to make the shape you need. It works easily, is free and readily available. If you use foam such as polystyrene you can't use polyester resin, the foam will dissolve.
7) Some plastics such as polyethylene won't work well with any resin unless a special surface treatment is done. This treatment consists of applying a coat of special "paint" that the resins will stick to. I cannot attest to the strength of repairs made using this technique.
8) Many people don't realize that the speed that epoxy cures can be varied with the addition of heat (faster), cold (slower) and by changing the rations of part "A" and "B". Be sure to speak to the manufacturer of the epoxy before changing ratios. Strength may or may not be affected. Epoxy can be put in the sun or microwave or refrigerator before mixing to obtain the desired effect. A heat gun applied judiciously will heat already laid up material and speed cure.
9) Cabosil, sand and pigments can be added to epoxy to obtain different properties. Cabosil can make it like a putty, sand can be a filler or add roughness for non skid surfaces and pigment can vary the color.
10) MEK (methyethylketone) can be used sparingly to thin epoxy and to clean up tools. It is similar to Acetone but evaporates slower.

I don't claim to be an expert in this field, my knowledge has come from experience and by speaking with epoxy and boat manufacturers. I hope this information is helpful.

09-20-2005, 09:44 AM
Hey, Thanks for all the tips and insight guys.

I may go looking for some other projects to hone my skills on. Uh.. wonder how a fiberglass lathe would work?