Stripping back to gel coat questions

[alolympic]

I am going to tackle at least the main legwork preparing my Rochdale Olympic for paint.
I am a complete novice but have started my research, and am happy to put in as much time as it takes.
The car was previously stripped back before the last respray, so I am not dealing with layer upon layer.

My first questions are....well, the start of the process.
Having read Miles Wilkins book, how to restore fibreglass bodywork, I just wanted to check the first steps,
He wrote to strip manually, use an orbital sander to go down to the first sealer coat.
He says to then use 220/320 wet and dry by hand. Wet.
A couple of questions on that.
What grade of sanding disc to use on a random orbital sander to start?
Is it still best to move to wet and dry by hand/with rubber block, even though these types of sanders do well to avoid sanding marks?
I am surprised about the advice to use wet paper to sand the sealer coat. I thought that would be dangerous, as gel coat is porous, and it is risky getting it wet. Maybe I have got the wrong impression from the book?

I appreciate I should really post this under the 'classic fibreglass' forum, but it doesn't seem to get much traffic....

Many thanks

[JeffreyB]

Having had many GRP bodied cars over the years I've had mixed results when repainting them. Bitter experience has taught me to get the body as dry as possible before painting so, in my opinion, that means avoiding wet flatting at all costs. I've recently had my Trident Venturer repainted and I expected the bodyshop to resist my instructions to dry flat only. I was therefore surprised when they said they haven't wet flatted for years. I'm no expert but my Gordon-Keeble was painted 25 years ago and only has a couple of micro blisters to show.
Good luck with the Olympic, I've always had a soft spot for them.

[Frankthered]

Would the presence of moisture cause this type of defect?

[JohnMcL]

Bodyshop told me there will be water in the fibreglass even if they only dry flat. So they leave parts in the oven for a few days before painting.

I discussed methods with the owner (a friend) as I wanted to do the job myself. He said the only advice he would give is that painting old grp is NEVER a diy job.

[plasticpig]

Frankthered said:

Would the presence of moisture cause this type of defect?

That looks like micro blistering caused by fiberglass osmosis.

As for wet sanding; when my Scimitar was resprayed they used heat lamps when sanding to ensure it dried out.



Obviously that's a bit OTT for a DIY job but you could do a small setup for use just where you are working.

[Fastpedeller]

IIRC Miles Wilkins book expressly says not to put a GRP car in a low-bake oven!

[Frankthered]

Fastpedeller said:

IIRC Miles Wilkins book expressly says not to put a GRP car in a low-bake oven!

I've heard some very strong conflicting views on this too!!

[gtmdriver]

If you heat up GRP, even with something as mild as a fan heater, you can change it's shape. I have used this technique to straighten out distorted panels in the past.

[alolympic]

So, I am glad I asked as wet sanding does sound like it could create all sorts of problems.
I presume dry sanding can get the required results, it wil just take a bit more patience, as the paper can clog, and I guess, will also generate dust, where wet sanding won't.
I will create a build thread I think, as I will probably need continual advice along the way.
...strangely excited although I probably have a few hundred hours of body prep ahead of me.
Thanks guys.

[Gareth9702]

It is worth noting that there is much more experience of maintaining, repairing, and painting fibreglass in the marine industry. If the micro blisters are from osmosis (and not just the consequence of moisture under the paint) then you should look to marine sources for advice on how to repair the problems. This book:

http://www.amazon.co.uk/Fibreglass-Boats-Construct...

is one of the standard references.

[Fastpedeller]

Gareth9702 said:

It is worth noting that there is much more experience of maintaining, repairing, and painting fibreglass in the marine industry. If the micro blisters are from osmosis (and not just the consequence of moisture under the paint) then you should look to marine sources for advice on how to repair the problems. This book:

http://www.amazon.co.uk/Fibreglass-Boats-Construct...

is one of the standard references.

Whilst I agree with the above, I'm not sure Osmosis is relevant to GRP Cars....... Correct me if necessary, but I understand that the "definition" of osmosis is that water has gone under pressure into the laminate and caused a chemical reaction with resin or catalyst which (for whatever reason) hasn't mixed properly. The 'water under pressure' bit is something which will be affecting boats under the waterline, but not cars (hopefully!)

[mateus]

I would say what you may be seeing is styrene escaping from the grp it won't be osmosis. I would suggest talking direct to professional boat sprayers for some advice. On the south coast desty marine or es marine and talk to Elliot.

Suffice to say wet flat shouldn't be an issue for gel or polyester resin the key will be the primer
Good luck.

[Gareth9702]

W

Fastpedeller said:

Gareth9702 said:

It is worth noting that there is much more experience of maintaining, repairing, and painting fibreglass in the marine industry. If the micro blisters are from osmosis (and not just the consequence of moisture under the paint) then you should look to marine sources for advice on how to repair the problems. This book:

http://www.amazon.co.uk/Fibreglass-Boats-Construct...

is one of the standard references.

Whilst I agree with the above, I'm not sure Osmosis is relevant to GRP Cars....... Correct me if necessary, but I understand that the "definition" of osmosis is that water has gone under pressure into the laminate and caused a chemical reaction with resin or catalyst which (for whatever reason) hasn't mixed properly. The 'water under pressure' bit is something which will be affecting boats under the waterline, but not cars (hopefully!)

It is not water pressure outside the laminate that is the problem. The excess chemicals in the laminate draw in water. This water becomes pressurised within the laminate and eventually causes the external blisters and cracked laminate that reveal osmosis.

[Fastpedeller]

Gareth9702 said:

W

Fastpedeller said:

Gareth9702 said:

It is worth noting that there is much more experience of maintaining, repairing, and painting fibreglass in the marine industry. If the micro blisters are from osmosis (and not just the consequence of moisture under the paint) then you should look to marine sources for advice on how to repair the problems. This book:

http://www.amazon.co.uk/Fibreglass-Boats-Construct...

is one of the standard references.

Whilst I agree with the above, I'm not sure Osmosis is relevant to GRP Cars....... Correct me if necessary, but I understand that the "definition" of osmosis is that water has gone under pressure into the laminate and caused a chemical reaction with resin or catalyst which (for whatever reason) hasn't mixed properly. The 'water under pressure' bit is something which will be affecting boats under the waterline, but not cars (hopefully!)

It is not water pressure outside the laminate that is the problem. The excess chemicals in the laminate draw in water. This water becomes pressurised within the laminate and eventually causes the external blisters and cracked laminate that reveal osmosis.

So is osmosis a problem with cars? or is it generally only boats that are a)subjected to more water and b)Have a lot more GRP, laid up in huge volumes and therefore maybe less likely to me mixed well and suffer excess chemicals in teh laminate?

[Gareth9702]

I do not know if cars get osmosis but I am happy to speculate.

Most boats spend at least 6 months a year sitting in water and only then "dry out" ashore during cold, wet winter months. This gives ample opportunity for water to be absorbed into the laminate (which is porous). Osmosis will then occur if the related chemicals are present. Fortunately, the chemicals are not present for the majority of boats.

In contrast, cars do not have the same exposure to water and will spend long periods in warm sunshine. This gives much less opportunity for water to penetrate the laminate and more opportunity for the laminate to dry. There is no reason why a car cannot suffer osmosis but the conditions of use make it less likely.

However, if a car was kept under a cover with a permanent layer of moisture between car and cover the chances of osmosis would be much higher.