Mounting new deck hardware, how to plug old holes?

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The Norwegian

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I'm swapping old aluminium clamps for some Spinlock deck hardware. Seems like I can't re-use too many of the holes, but luckily the new parts will cover the old. What should I do to fill the holes, and how should I do it?
I'm thinking either I could put something underneath that won't bond (some types of plastic?), then fill it up with epoxy, or just use some kind of Sika sealant.
In the new holes, which of the two should I use? West systems recommend using epoxy for maximum strength, but I can imagine that would render the new bolts pretty irremovable, and does it really matter? It's not a racing boat, even though I like sporty sailing.
Any other advice and lessons learnt would be appreciated. Thanks!
 
For old holes drill them out a tiny bit so the holes are nice and clean all the way through. If you can countersink them slightly so much the better. Then stick some tape underneath and fill them with epoxy, removing the tape when dry. If your new fitting cover the old holes you can leave the epoxy slightly proud and sand flush when dry. If any are visible leave the epoxy slightly low and when it is dry finish off with some flowcoat, leaving this slightly proud to be sanded flat.

Out of sight epoxy can be sanded with 240 wet paper. Visible flowcoat needs to be done carefully with 240 wet followed by 600 wet and then polished.

To seal the new holes use Sikaflex, not epoxy.
 
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Thanks a lot, that was very descriptive, answered everything I needed. Countersink means to cut a conical shape slightly? This should perhaps be done with a dremel tool? I am a bit afraid that there will be rough edges when using that. Do you sand by hand?
 
The Norwegian said:
Thanks a lot, that was very descriptive, answered everything I needed. Countersink means to cut a conical shape slightly? This should perhaps be done with a dremel tool? I am a bit afraid that there will be rough edges when using that. Do you sand by hand?
Click to expand...

You can countersink these particular holes with a drill bit. Pick a drillbit a few sizes bigger then the holes and carefully just start to drill the hole.

If you're just repairing small holes it won't be hard to sand by hand. Cut the sandpaper into four and wrap around a small piece of wood if you need to keep a flat finish.
 
I fitted a new cable port and had 4 20 mm holes to fill where old deck glands with threads and been screwed into tapped fibre glass holes. I used a burr type drill to roughen the hole up and clean out the threads, used sandpaper to chamfer, taped thin ply over the holes below, filled with thickened epoxy that was still runny enough. Once the epoxy was in I poked it for a while with thin wire which appeared to bring up bubbles that were trapped. When the level dropped I topped up with fresh epoxy. Later I just pealed the ply when the epoxy had gelled up, about 1 hour later, but was still soft enough to poke a finger nail into, it came away surprisingly easy. Next job is to finish fair and paint over the holes.
 
wooslehunter said:
Don't use a drill to countersink. It will grab & break out the surface unless you are very careful. Use one of these http://www.screwfix.com/p/countersink-carbon-steel-12mm/14156 or http://www.screwfix.com/p/countersink-hss-12mm-round-shaft/11311

It will also cut the correct angle for countersink screws so keep it for the future. A drill cuts the wrong angle for countersunk screw heads.
Click to expand...

That's why i said "for these particular holes", because he's not countersinking in the normal sense of the word. I also said to be careful, very light pressure and a quick burst from the drill works fine.

The countersink tools you linked too require just as much care when used with GRP. If you really want to buy new tools one of these is a better choice :

http://toolguyd.com/keo-countersink-drill-bits/
 
The hardware being mounted to the deck will not require countersunk holes to accept the head of a countersunk screws. It is likely that the hardware will bolt flush with the deck and the screw or bolt head will be sitting on the flange / body of the hardware.

The purpose of countersinking a hole when a flat face flange is going over the countersunk hole is to allow a bead of sealant to seal around the shaft of any screw / bolt as an additional security against water finding its way through / into the hole, normally a hole that is through drilled for a bolt below. The method particularly is used when using a non setting material that is rolled into a thin sausage and placed around the hole. When the flange face clamps down the bead squeezes against the countersunk sides and bolt shaft effecting a seal.

For blind tapped holes, Gudgeon Brothers recommend using epoxy to both lock in and seal the screw. They advise that a soldering iron applied to the screw head can be used to soften the epoxy and remove the screw. In tests they have conducted the strength of the hole with fastener screwed in has exceeded the yield strength of the screw. For items that you may want to remove more frequently they recommend that you work the screws as the epoxy sets so that the screw never bonds properly but the epoxy still gets to repair the tiny fractures that a self tapper might induce in the hole. Many folks do not set screws into epoxy and they get adequate strength.

Anyway, if it through bolted I would not fill the holes with epoxy as all the strength is achieved by the nut, bolt and backing pad.

Compass Marine explains it better here and also show that countersinking helps reduce or eliminate spider cracking from straight drilled holes in GRP http://www.pbase.com/mainecruising/rebedding_hardware
 
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