Patching GRP in the cockpit

[Tam Lin]

Following on from the question about applying fibreglass to plywood. I have taken instruments and controls out of the cockpit and am left with some holes. Which is the best way to cover them? I have thought of filling them with epoxy / GRP / flow coat, putting a plywood patch over them or even a stainless grille. I want it to look nice, what have other people done?

 

[prv]

Just done exactly this.

I started by wax-polishing a bit of conti-board to make a non-stick surface, and laid up circles of mat and resin (use polyester, there's no need whatsoever for epoxy in this job) the right size for each hole. Actually I made them a bit over-size, and trimmed them to fit with scissors while at the "green" stage of curing. I think I used three layers of mat, can't remember what weight. Use a paddle-roller to consolidate - this applies to the whole job - just stippling with a brush really isn't good enough.

I then bevelled the edges of each hole using a power-file, and also took off the gelcoat for a short distance around it. Other tools will work too, but I find the power-file best for rapidly re-shaping GRP. I use the metal-cutting belt.

I fitted a short wooden batten across the back of each hole to support the disks I'd made. Where I had access from the back, I waxed the batten so it wouldn't stick. Where I didn't have access except through the hole itself, I attached the batten using screws through the GRP either side of the hole (just outside the bevelled area) and instead of preventing sticking I actively stuck it to the back of the disk with a dab of resin. This meant that when I removed the screws at the end, the batten would stay in place and not rattle around inside the moulding.

I then cut out more circles of mat to fill the holes. I was filling quite large holes in a vulnerable cockpit side, so I used five layers of mat making eight in total with the already laid-up disks. The largest circle should be the size of the outermost extent of the bevel, the smallest a bit smaller than the disk. Cut them all out in advance, you don't want to be doing it while messing about with resin.

I then set the disks in the holes, supported by the batten, and painted the whole area with resin. Stuck the largest circle of mat in place - you start with the largest so that there's an unbroken layer tying the whole job together. Then work down through the sizes, applying resin and consolidating it with the roller - to stop the disk pivoting on the batten while doing this, I pressed down on the opposite side with a pointed bit of wood.

Ideally you'll end up with a flat repair that perfectly fills the bevelled space and is level with the existing GRP (not the gelcoat) surrounding it. I usually have to apply the power-file in a couple of places after it's set, to bring down some raised areas. Then you fill the remaining depression with gelcoat. Sounds easy in theory, but I usually end up having to do several rounds of sanding and filling. Don't add wax solution to a layer which is not going to be the final layer - nothing will stick to it! In theory you'd add wax to the final layer, but this time I found I'd run out (or all the styrene had evaporated?) so went without. Since I was sanding it anyway, the initial surface tackiness didn't matter.

I generally sand down to 320grit then wax it to feel smooth, but a perfectionist would probably go down to wet&dry and polishing compounds etc.

Pete

 

[Scubadoo]

I used a screw and white cover/cap for the engine bay once when I relocated a fitting and looks fine, maybe an idea or as a temporary measure.

 

[Tam Lin]

Pete, Thanks for taking the time to give this helpful and detailed reply!

 

[oldsaltoz]

If you only want to hide the holes:

Put some tape under each hole to capture the dust when you countersink the holes. This will provide a bigger contact area for the repair and provide you with a filler for the gel coat mix, the older gel coat will match the existing.

Tape over the outside and fill from the inside with a tape first and a small hole in it.
You can use talcum powder as a filler for this part. when filled, pull the tape off and wipe any residue off with a bit of Acetone.

Return the outside mix your gelcoat using the shavings as mentioned above, slightly overfill for later sanding and polishing.
Note, make sure you use a block with the sanding paper.

Good luck and fair winds.

 

[laika]

IIRC from an earlier thread prv's task was constrained by lack of access to the rear of the holes. If you do have decent access (as I did when patching up old speaker holes last year) the task can be done "the other way round" (working from the inside, gelcoat first) which may be easier: Can you get at the holes from behind?

Obviously the best thing to cover up an instrument hole is probably another instrument if you're looking for an excuse to buy new toys

 

[westhinder]

IIRC from an earlier thread prv's task was constrained by lack of access to the rear of the holes. If you do have decent access (as I did when patching up old speaker holes last year) the task can be done "the other way round" (working from the inside, gelcoat first) which may be easier: Can you get at the holes from behind?

Obviously the best thing to cover up an instrument hole is probably another instrument if you're looking for an excuse to buy new toys

I may have to do this, so I'm interested in the way you went about it. With what did you cover the hole, i.e. What was the substrate for the gelcoat? And how did you match the colour of the new gelcoat with the surrounding gelcoat, which must have changed from its original presumably white colour?

 

[laika]

I may have to do this, so I'm interested in the way you went about it. With what did you cover the hole, i.e. What was the substrate for the gelcoat? And how did you match the colour of the new gelcoat with the surrounding gelcoat, which must have changed from its original presumably white colour?

The canonical substance is formica which I gather that "normal" people have loads of offcuts of kicking about but I couldn't find any, nor could I buy a small amount of it so I ended up using some acrylic which may be dodgy from a chemical point of view but worked. Something smooth and shiny is what you need. I followed my book which suggested covering the shiny side with release wax then pva release agent. Someone I think told me the latter was unnecessary and I'd now be inclined to agree, especially as I didn't let it dry enough before starting and ended up with blue streaks in the gelcoat which I had to grind out and re-fill.

So...wax on the formica (or other shiny stuff). Bevel the *inside* of the hole, screw formica over the *outside*of the hole with the waxed shiny face inwards.

Now the book instructions say to paint gecoat over the wax but this turns out to be easier said than done: I had to kind of stipple the first layer on. Second layer was easier and third layer I could paint. When sufficient gelcoat applied, start the layup using pre-cut CSM as prv described but starting with the smallest and working up sizes.

Despite the feeling that it really wasn't going as smoothly as the textbook implied it would, it looked ok when I took the acrylic (which should have been formica) off and cleaned off the blue pva and wax. Had to dig some bits of blue and some bubble holes and re-fill. Then wet and dry used wet up to 1200 and polished with the rest of the cockpit.

Didn't try and colour match but the areas of old gelcoat around the repair where I'd taken the surface layer off with the wet and dry were closer to the new white than the surrounding gelcoat.

A year on you can see where something has been done if you're looking for it but it's down at ankle height in the gap between the wheel and the back of my (centre) cockpit. Certainly a lot less noticeable than the big acrylic discs which covered the holes when I bought the boat.

 

[Tam Lin]

H

IIRC from an earlier thread prv's task was constrained by lack of access to the rear of the holes. If you do have decent access (as I did when patching up old speaker holes last year) the task can be done "the other way round" (working from the inside, gelcoat first) which may be easier: Can you get at the holes from behind?

Obviously the best thing to cover up an instrument hole is probably another instrument if you're looking for an excuse to buy new toys

Yes, I have got access from the inside. I won't be putting new instruments in the holes as they are all down near floor level.

 

[laika]

Yes, I have got access from the inside. I won't be putting new instruments in the holes as they are all down near floor level.

Then I'd be tempted to do it the way I did which was what my various books seemed to advise, although I'd be interested to hear other opinions as I'm still learning

If working inside then do do make sure there's plenty of ventilation: I gather styrene really isn't nice and as well as leaving all the windows and hatches open I was sticking my head outside for breaths. Cover everything that there's a change the resin might drip/splash onto. Wear goggles and the best mask you have to sand down the scraggy bits on the inside of the layup once set.

 

[prv]

Good point about the inside access - the gelcoat work is easier if you have a "mould" surface like Laika's acrylic, rather than having to sand and fair it. Any reasonably shiny surface will do, I've used all sorts. A quick buff with wax polish if you have some handy will guarantee it doesn't stick, but it's not really necessary. I only use the blue PVA on female moulds where you can't peel the surfaces apart.

Pete

 

[lw395]

Any piece of card, plywood, plastic will serve as a mould, I sometimes cover them in parcel tape or polythene to get a smooth surface.
Even if you're working from the top, pushing a flat mould on top of the work can reduce the amount of sanding.