Outer Limits 4x4 Board

Hey guys!

I'm after some first hand info on using flowcoat. My plan is to repaint my fibreglass hydrobath a nice white, the bath is currently a faded & porous blue with some wear patches that have no colouring.

I've done a bit of research on boat forums about the process, but I still would like to maybe run this by someone here who has done this before.

So basically from what I have read I need to sand & acetone the existing gelcoat/flowcoat (not sure which it is on there at the moment but I'm assuming flowcoat because the outside looks to have been rollered on, and the inside possibly sprayed as it is smooth). I've concluded that my next step is to paint the flowcoat on using the roller for the outside (because it's already texured looking) leave it to cure for 3 hours or so and sand it back using appropriate grade papers, then finally cut & polish? Or would I not need to sand the outside as it is meant to have the textured look?

For the inside I plan to spray it, am I right to think I will need to thin the flowcoat using styrene? If so is this something I can approximate the viscocity as I go, or, should I use a more specific measurement ? If I were to then spray it on the prepped surface, do I just apply one thick coat or multiple?

The inside is much more important to me to get right as I don't feel like forking out another few k on a new bath if I happen to fuck it up. It must be water tight and fairly durable as I will have an alloy frame inside the bath that sometimes bounces around in there when I'm driving, and also Jeremy scratches it with his toenails sometimes when I'm bathing him, no pics sorry.

Appreciation in advance for any useful advice.

When my partner had her dog grommimg business and the hydro bath need a bit of repair inside the bath where she use to put in an insert in to wash small dogs it wore away the fibreglass till it was a clear color. I just went to super cheap got a tin of gel coat and put that in the corners and sanded it.

Flowcoat and gelcoat are essentially the same thing with differing quantities of styrene monomer.

Prep: Sand any surface to be coated with at least 180grit paper to allow the chemical bond to take place. Personally I use 100 grit to save on time but still achieve a quality finish.

You'll need to thin your gelcoat with around 10% styrene (measured by volume is fine). This should give it a consistency slightly thicker than milk. If you were to put a paddlepop stick in to stir it, it should run off the stick in a thick stream and not drip as if it were water.
You'll need a gelcoat spray gun due to the thickness. A HVLP gun with a nozzle of 1.2mm or bigger will do at a pinch.
If you're having trouble getting it through the gun, try adding 5% acetone.


Wipe any surface to be coated with acetone prior to spraying.

Catalyse with 2% MEKP catalyst. 1.8% is the ideal ratio and 3% is the maximum recommended for gelcoat applications. At this time of year in colder areas you may well need to use it.
You can always use a heater to increase ambient temp's to suit or use a heat gun after spraying to speed up the tacking off.

Gelcoat should be sprayed at an ideal thickness of 0.5mm. This can be visually identified by watching for when the spray job stops looking like fine mist and looks "wet". Thicker coats don't hurt and can be useful in high trafficed/easily damaged locations or if you don't have delicate hands for the wet rubbing process. Generally a single coat only is sprayed. If you need to recoat, you will need to scuff the surface again prior to spraying.

Curing time at 25degrees C is around 3 hours before you can wet rub.
Once the gelcoat has tacked off and is ready to be rubbed, wipe the surface over thoroughly with a rag soaked in acetone. This will remove the stickiness from the gelcoat finish and allow you to sand easier without clogging of the paper. This stickiness can be prevented by the use of "Wax in Styrene" which is a parrafin wax disolved in styrene monomer, but using it will complicate any future repairs you will do as it is hard to coat over.
I spray with a fairly flat, orange peal free finish so I wet rub with 800 grit first then 1500 followed by a liquid buff and polish.
The norm would be to wet rub with 400, 800, 1200, 1500 and aquabuff (2000 grit liquid). Very heavy orange peal (from too low an air pressure or spraying too close) could need a dry rub with 320 or similar to remove.

Great info thanks mnemonix. The solution I'm looking at buying has the wax already added, the info on the site I was thinking of buying it from reccomends not using gel-coat as it needs to be sealed from air for it to cure. Where as the flowcoat is gel-coat with the wax already in it so it will cure without being sealed. It also comes with the catalyst. They are asking about $35 delivered for a litre of it. Do you think this is reasonable or is there somewhere I could buy it cheaper? I don't know but do regular paint stores sell it or is somewhere like supercheap the go?

Also forgot to ask, how much might I need for the bath, just a rough idea? Seeing as it's inside and out (but not underneath) I estimate it's probably 8-9 mtrs square all up. I suppose that depends on how much I thin it as well.

ledgend80 wrote:When my partner had her dog grommimg business and the hydro bath need a bit of repair inside the bath where she use to put in an insert in to wash small dogs it wore away the fibreglass till it was a clear color. I just went to super cheap got a tin of gel coat and put that in the corners and sanded it.

Ok cool! I didn't realise supercheap sell it too. Will check them out, ta

I would be very surprised if you used more than 1.5 litres for a job that size. Because of the thin coating it does go quite a long way.
Have you considered a 2pack finish? Gelcoat isn't the most hard wearing finish and will need recoating in the future to keep it's "new" look.
Properly applied 2pack is damn near bulletproof in comparison.

mnemonix wrote:I would be very surprised if you used more than 1.5 litres for a job that size. Because of the thin coating it does go quite a long way.
Have you considered a 2pack finish? Gelcoat isn't the most hard wearing finish and will need recoating in the future to keep it's "new" look.
Properly applied 2pack is damn near bulletproof in comparison.

Depends on how much moisture & flexing of the fibreglass it can withstand. I thought gel-coat because that is what is originally used on hydrobaths/ spas.

I can't help with how to use it, but we use heaps of this at work repairing semi-trailers, we buy our stock from Nuplex Composites, if there is a branch somewhere near you.

Ok will research them tommorrow when I do my ring around.

Most boat chandelerys like Bias and whitworths sell it, so do fibreglass suppliers like FGI, preperation is the key make sure all areas have been sanded and wiped clean with acetone.

Bath has been sanded, acetoned and the outside has had the flowcoat rollered on. It came up good except for some areas where there was residual silastic I thought had been all removed. Damn that crap is impossible to remove!! Though I did manage to fix it after all.

The consistency of the flowcoat was runnier than expected so I did have to wait a little while for it to thicken, but once it thickened I had to work reasonably fast to get it on.

I am happy with the job done so far it's a lovely finish. I used a whole 1ltr can doing the outside of the bath (probably 4 metres sq) So I will need to buy more for the inside as well as some styrene to thin it for the spray gun.

One thick coat was all that was require to cover the blue that was underneath though if I were to do it again I would apply 2 thin coats instead, it uses less paint this way & seems to give better coverage.

Not sure if the same thing will work for what you are doing, but for automotive paints you can get an additive to help paints deal with silicone residue, it stops the paint from getting "fish eyes" or small round holes in the paint where silicone residue breaks the surface tension.

Lots of polishes have silicones in them, so would be a good chance you won't be able to see any sign of silicone on the surface.

80's_delirious wrote:Not sure if the same thing will work for what you are doing, but for automotive paints you can get an additive to help paints deal with silicone residue, it stops the paint from getting "fish eyes" or small round holes in the paint where silicone residue breaks the surface tension.

Lots of polishes have silicones in them, so would be a good chance you won't be able to see any sign of silicone on the surface.

I learned about it after I'd applied the flow coat and it started to seperate Actually I was wondering how I will go spraying in the resevoir where the water and heating element lives, I wasn't exactly neat & tidy when I re-siliconed around the bits and pieces in there.

I've been told not to spray on the flow coat for the inside now, spoke to a guy from a boat repair shop he reccomends brushing it on then sanding it smooth, sounds like a lot of extra work to me but if that's what the pro's do I'd best listen. Picking up some more flow coat from this guy tommorrow, it hopefully is cheaper this way as well.

Oh well, I'm off to do some more sanding!

Silicone is horrid stuff. I try to use polyurethane when possible, ie sikaflex. It's mongrel stuff too, but at least you can paint over it.

Brushed gelcoat is horrendous to sand compared to sprayed, but the finished result is the same. You'll use more gelcoat also.
You'll almost never find a pro who brushes, mainly for the neatness, low material use and speed of spraying.

you sort out your rust troubles ferog?

van was it?

Cruza, I wrote off the other van in an accident so I have another, it's currently getting a half cast of fibreglass with proper drainage, sealed all around and then up to the roof I will fish oil or something and use the skin reccomended to protect it some more, plus extractor vent. Hopefully that will make it a bit more rust proof this time?? Given the last van had already had rust, this one doesn't, it's a better start.

Well I started out strong the outside of the bath went well getting it rollered on.

I was told to brush or sponge roller the inside by a fibreglass professional, all good in theory, I thought I'd be smarter and buy one of those sponge brushes, like what you use in the corners but this was a big flat one. It worked well for the first few strokes but after it started to thicken it became a bit of a desperate attempt to get it all on at once. I ended up using a small paint brush (all I had) to quickly get the thickening paint on. (I did do it in 500ml lots but it still was a mess to use)

So for my success for the outside the inside is a pretty big let down. The only positive that may save it is because the whole thing is white, it is so blindingly stark I don't think anyone will notice unless they have a close look, then, they might vomit at the lumps and scrapes that are now part of my new fancy custom gel coat job.

Fuck it!

(Ps- is there an easy way to remove flow coat from a concrete driveway, the cat & human hair? )

before sanding and polishing/ tidy up. Looks great in photos!

Oh btw, I used approximately 2.5 litres of flow coat, I really needed another 500ml- 1 Ltr more to cover it all perfectly.

Mixed at a ratio of 2% mekp to flow coat. And a bucket load of acetone to clean up & prep.

Cheapest place I found to buy it with the catalyst was ebay & that's including postage.

Looks good from here

for removal from your hair, your only option is clippers, #2 all over should do the trick


Same deal to fix the cat, but make sure you have video camera rolling while you clipper it, and put it on youtube

Oh, and your driveway is now phwucked!




I don't know how you can fix that. You could try some hydrochloric acid, but be vewy vewy careful, its nasty stuff.

Let me ask the guys at work tomorrow how they clean it up, all the brushes and rollers we use have a very, very short life... If whatever we use isn't too nasty, I can send some your way.

Cleaning is done with acetone. If it's already cured on your driveway you won't easily remove it.

In a previous life I was a fibreglass fabricator specialising in gelcoat detailing and finishing.