Drilling Holes in Fiberglass

[70_Steve]

I just picked up a pair of never used fiberglass fenders as another tentative step towards actually repainting my Bronco. These fenders are of unknown origin and manufacture, not that this makes too much difference. These fenders have indentations molded into them, where the bolt holes go.

In thinking about the process of fitting them, obviously I will have to drill through the fiberglass at these locations. Which raised a couple questions...

1. What is the best type drill bit, tool, or process to drill through the fiberglass without causing damage to the surrounding area?
2. After drilling, should the area around the hole be finished with something (glass resin, etc)?

Thanks for any words of wisdom and experience!

 

[Glass Pony]

Disclaimer: I am no expert.
Hole saw is good for big holes. I have also had good luck with those step drill bits with the multiple diameters on one bit.
If I don't have the size I want in those types I'll take a regular twist drill bit and knock the sharp edge off so the bit "scrapes" instead of "cuts".
Also a slow feed.
As for sealing, if the fiberglass doesn't have a core I don't worry about it.

 

[bmc69]

Brad point bits...they are the bomb.

https://www.mcmaster.com/#2828a75/=15htl1f

 

[Glass Pony]

BTW, sometimes the indentations molded in are not quite in the correct position so check first before drilling.
Ask me how I know this.:-[

 

[pcf_mark]

Yes yes yes - never trust the indentations for anything always fit them five times over before drilling.

I use a die grinder / air grinder with a bit in it. Cuts through slow but the edges stay nice and clean. You need to hold it firmly in place to go straight through. You will need this tool anyway to clean and trim edges so I would get one if you don't have one.

I use electric because they are quieter but here is one from Sears
http://www.sears.com/craftsman-die-...SellerId=Sears&prdNo=2&blockNo=2&blockType=G2

 

[70_Steve]

Brad point bits...they are the bomb.

https://www.mcmaster.com/#2828a75/=15htl1f

Thanks bmc69. Was thinking I'd seen something like this before. I'll add this to my McMaster order...

 

[70_Steve]

Yes yes yes - never trust the indentations for anything always fit them five times over before drilling.

Yes, planned on fitting these up before doing anything, including drilling holes!

I use a die grinder / air grinder with a bit in it.

Got the die grinder. What type bit did you use?

 

[70_Steve]

Thanks guys! Got a couple good suggestion on making the holes.

So there's no need to "finish" the hole, other than primer and paint? I guess the holes are just like any other edge of the fiberglass... Hmm... seems I was overthinking things... again...

 

[grant_71]

Thanks guys! Got a couple good suggestion on making the holes.

So there's no need to "finish" the hole, other than primer and paint? I guess the holes are just like any other edge of the fiberglass... Hmm... seems I was overthinking things... again...

On an automotive body as long as you are not drilling into/ through a support that is not fiberglass (wood or steel) I would not worry about it

If it is a marine environment I would say you need to seal the spots you drill, but on a set of fenders, nothing to worry about.

Primer and paint are fine, the only "finishing" you may need to do are any raw edges of glass get a skim coat of resin just to keep from getting those nasty 'glass slivers.

 

[Pops68]

I would 'firmly' clamp a piece of pine or plywood to the back side before you drill the hole....will help with tear out/breakage on the back side. Probably a minimum of 1/2" thick or thicker.

Pops

 

[bmc69]

I would 'firmly' clamp a piece of pine or plywood to the back side before you drill the hole....will help with tear out/breakage on the back side. Probably a minimum of 1/2" thick or thicker.

Pops

Those brad point bits I posted the link to eliminate the need for all that...they drill very clean holes in FRP...with no tearout.

 

[pcf_mark]

I used a bit meant for steel - usually red. The grey ones are too fine and take forever. The grinding gives you a nice clean edge no furry fibers. I have restored several Corvettes and use it for everything.

 

[Rustytruck]

Chamfer the gel coat after you drill the hole its less likely to crack. I also like to tape over the hole location before I mark and drill. Its also wise to use a flat washer between the bolt and the fiberglass so the bolt clamps up on the assembly rather than the bolt twisting into the surface. A strip of Butyl tape from the RV suppliers between the fiberglass fender and the fender liner will support the fiberglass better and make finish paint hold up better all along the fender mounting points.

Look up brad point drills for drilling fiberglass.
http://www.sears.com/cm-bradpointb/...KQghyEExdkS2si8aLpH_bxoC07vw_wcB&gclsrc=aw.ds

https://www.mcmaster.com/#chamfering-countersinks/=15hxb61

http://www.rvpartsnation.com/rv-ext...xREdd2ZuDGzT7qo1kinoqrRlDUNFIcyyuPhoCvSrw_wcB

 

[70_Steve]

Chamfer the gel coat after you drill the hole its less likely to crack. I also like to tape over the hole location before I mark and drill. Its also wise to use a flat washer between the bolt and the fiberglass so the bolt clamps up on the assembly rather than the bolt twisting into the surface. A strip of Butyl tape from the RV suppliers between the fiberglass fender and the fender liner will support the fiberglass better and make finish paint hold up better all along the fender mounting points.

?The butyl tape is an idea. I had thought of using something like self-fusing electrical tape.
https://www.mcmaster.com/#electrical-insulating-tape/=15hxf2q

 

[EricLar80]

I've used a spade bit with really good results. Make sure you put painters tape down first.