Advice sought on screw fixing to inside of coachroof

[Guest]

Can anyone suggest a strong filler for a 4mm diameter X 8mm deep hole in my grp/wood coachroof. It needs to be drillable to take a woodscrew under tension. A quick setting filler would be a bonus.

A plastic rawlplug bedded in Araldite was nearly, but not quite, man enough for the job - it came out just as I thought I had succeeded.

 

[Guest]

Drill the hole clean using a 5mm drill then use 5 minute setting epoxy to fill the hole. Insert the screw ( not under pressure ) after 20 mins the screw will be fixed stronger than the original. Alternatively, after drilling out the hole, epoxy in a 5mm dia hardwood plug. When the epoxy is set screw in the screw.

 

[Guest]

Iain's suggestion of using epoxy to glue in the screw is undoubtedly the best that does not require further reinforcing.

But if this is still not strong enough for a heavy load, and the hole is vertical, I'd be tempted to drill right through the roof and fit an M4/M6 pan-headed stainless bolt with a washer to spread the load. Rebate the hole outside, bed in the bolt with epoxy and cover the head with gelcoat for a watertight and reasonably invisible external finish.

 

[Guest]

Many thanks for the quick replies.

Unfortunately setting the screw in epoxy is not practical as the screw will be under tension from the beginning. It is one of several holding a thin fabric covered plywood internal lining to the coachroof. The tension is caused by the slight concave shape of the panel.

The hardwood plug set in epoxy could well work but is really not that different from the plastic rawlplug set in Araldite which I have already tried.

The bolt through the coachroof is really overkill for a fixing that has held fast for three years but only failed over the past winter, presumably because of damp in the wood between the inner and outer grp. An additional problem would be that the outer surface has been painted with deck paint and I do not have any to touch up the filler.

One idea is to hollow out the woodwork between the grp and fill the hole with a tough epoxy type filler that can be drilled when hard to hold the screw in the same way as if I was screwing into wood. The only problem I do not know of the relevant trade name.

Any suggestions please.

 

[Guest]

Alan, I think that you would find that a hardwood plug would be much better than the plastic rawlplug as the epoxy would adhere to the wood, unlike the plastic rawlplug and araldite. When gluing in the wooden plug use a stiff epoxy mix with coliodal silica and microfibres. If you do not stiffen up the mix the epoxy will not be gap filling and the bond will not be so strong. Also when inserting the screw, screw it in 3/4 way then remove it and dip it in epoxy before reinserting it this will give you a very strong bond.

 

[Guest]

Re: Inserts

I have roof panels originaly fitted in the same way. I quickly discovered that these needed to come down every time access to wiring, coach roof fittings etc. was required. Result? after a while the original wood screws had "worn out" the holes in the roof. I decided that machine screws were a better bet. I discovered some aluminium alloy inserts these have a coarse wood screw thread externally and a machine screw thread internally. They come in a variety of sizes, but a 4mm internal thread is about 10mm external.

The roof was drilled out where the original screws were fitted the inserts screwed in with a little epoxy just to make sure. The roof panel is then held up with the 4mm screws screwed through the original screw holes in the panel into the inserts.

 

[Guest]

Rather than woodscrews...

Consider using stainless self-tapping screws. I have just finished the headlining of my Centaur using three panels of plywood covered with carpeting. The curve of the Centaur cabin roof is quite pronounced. I almost invariably use these self tappers instead of normal woodscrews for two good reasons : 1. for some arcane reason they are sold at a fraction of the price of stainless woodscrews and, 2. the thread is parallel (apart from a short point) and is usually sharper and deeper than that of a 'normal' screw, therefore it's easier to drive in and the grip is much stronger. I pre-drill at the core diameter to about 3/4 of the depth. For my panels I used 3.5mm screws, drilling with a 2.5mm bit.