Painting a GRP body

[Kentish]

Should I pre-fit the body first and then remove it for spraying, or leave most of it fitted?

There are some large sections and I guess these could easily be damaged (paint that is) whilst refitting again later.

I understand that I have to heat the body up to around 50 degrees C whilst fitted to the chassis, then let it cool slowly to destress everything and release chemicals from the GRP.

I'd like to coat the inside raw GRP finish with a black rubber coating to reduce noise and give a nicer finish and avoid stone chipping and star crazing. Will it be OK to do this prior to fitting and heating the body?

I don't want to have to heat it all up then remove it all from chassis to spray inside then refit it all again, that would be lots of extra work.

TIA

Edited by Kentish on Tuesday 20th June 11:00

[steve_d]

You need to do as much of the body fit as possible before paint. Electric drills around new paintwork is not a good idea.

The preheat of the body is to release air bubbles trapped in the gel coat. I have not come across the heat being used to release stress. If there is stress in the body when bolted to the chassis it has not been fitted correctly. If you leave it like that and expect heat to solve the problem then in a year or less you will start to get star cracks due to the stress.
I would plan to remove the body or at least down to the central tub if your kit has one.
The rubber coat should be done last after the paint has been oven cured. You may also need to do more of this when the panels are back on as you can then seal the seams underneath which will make a far better job overall.

Steve

[splatspeed]

boyd coddington and the crew on american hotrood allways paint the body off the car after a fitting session

they are doing a 55 corvette i think on discovery home and leisure at the moment good to watch to see how they do it

sean

[Kentish]

The car has two main body parts, these being the front and rear ends and they are fairly big. I'd be rather nervous about transporting those around after spraying and not fitted to the chassis, they will get damaged easily due to their size. Plus when they're heated prior to spraying they'll distort and no longer fit the body. So I think these will have to be fitted and remain on the chassis.

There is also a rear tailgate surround, a bonnet and outer door panels and a rear wing (this needs mounting and the join filling).

If the door panels aren't fitted they also distort when heated. The bonnet has to be fitted whilst it's heated and the corners held down to prevent distortion, Tailgate glass has to be fitted for the same reason.

I am going to do a full fit first anyway and I think I'll leave most of the car together for the pre-heat, then let it cool down and remove the bonnet and tailgate but leave all the other panels on the car and mask well including all apertures and anything that should have any overspray, then prime and spray and lacquer.

I'll make sure I pre-fit everything like mirrors, lights, wipers, rear quarter glass but remove them for painting and refit later where possible.

This is going to be fun

[Tony427]

I'd leave the bodywork on the car in gelcoat and run it around for a year or two before you paint it. This is the general concensus of opinion in the Cobra fraternity if you want a long lasting and crack free paintjob.

If you really want to paint immediately you will need to hot bake the car for a weekend or so in a paint oven to really get all the stresses out and to make sure the GRP has moved/flexed/relaxed as much as possible.

Cheers,

Tony

[Vospers-Alan]

This came from Nexia paints before we painted the cobra
GRP PRIMING RECOMMENDATION

WARNING
BEAR IN MIND THAT THE SUBSTRATE IS HANDLAID FIBREGLASS.
YOU SHOULD NOT EXPECT TO CREATE A “METAL” TYPE SHAPE OR FINISH WITH A PRIMER.

ATTEMPTING TO BUILD UP THE PRIMER THICKNESS TO SUCH LEVELS WILL RESULT IN CRACKING. IT WILL ALSO NOT PREVENT THE RIPPLES FROM REAPPEARING OVER A NUMBER OF MONTHS.

INITIAL REPAIRS
IDENTIFY LOW AND HIGH SPOTS IN GRP USING CORRECT SIZE BLOCK SUITABLE FOR THE AREA IN QUESTION, P180/240 GRADE STRIPS WILL BE SUITABLE FOR THIS PROCESS. REPAIR ALL DAMAGED AREAS AND INPERFECTIONS USING GOOD QUALITY POLEYESTER BODY FILLER.
ONCE ALL REPAIRS HAVE BEEN CARRIED OUT PREPARE REMAINING AREAS USING AN ORBITAL SANDER AND P240/320 GRADE DISCS.

PAINT RECOMMENDATION
DEGREASE WITH P850-1402 SOLVENT SPIRIT WIPE. APPLY THREE COATS OF P565-888 (SURFACER MODE) AND ALLOW TO AIR DRY FOR 24HRS. (DO NOT BAKE) APPLY GUIDE COAT TO PRIMED AREAS, BLOCK DOWN USING BLOCKS AND P180 GRADE STRIPS.
IN INACCESSIBLE AREAS FLAT BY HAND. THIS FLATTING PROCESS IS USED TO SMOOTH MINOR INPERFECTIONS IN THE GEL COAT, THIS WILL PROBABLY REMOVE A MAJORITY OF THE PRIMER PREVIOUSLY APPLIED.
RE-PRIME AREA WITH A FURTHER THREE COATS OF P565-888 SURFACER MODE AND ALLOW TO AIR DRY FOR 24HRS (DO NOT BAKE) GUIDE COAT PRIMED AREAS AND BLOCK DOWN WITH SOFT FACED BLOCKS AND P320 GRADE STRIPS AFTER ALL SIGNS OF GUIDE COAT HAS BEEN REMOVED RE-GUIDE COAT AND SAND USING ORBITAL SANDER WITH SOFT BACKING PAD AND P500 GRADE DISCS.

FINAL STAGE
DEGREASE WITH P850-1402 SOLVENT SPIRIT WIPE. APPLY ONE TO TWO COATS OF P565-357 OF THE CORRECT SPECTRAL GREY FOR THE COLOUR THAT THE VEHICLE WILL BE REFINISHED IN. THIS WILL GIVE A HIGHER GLOSS FINISH WHICH WILL SHOW ANY PROBLEM AREAS MISSED, ALLOW TO AIR DRY (DO NOT BAKE) IF HAPPY WITH RESULTS RE-SAND WITH ORBITAL SANDER WITH SOFT BACKING PAD AND P500 GRADE DISCS APPLY TOP COATS AS REQUIRED, ALLOW TO AIR DRY (DO NOT BAKE).

POINTS TO NOTE
ALL SANDING TO BE CARRIED OUT DRY (DO NOT WET FLAT)
WHEN BLOCKING BODY TO HIGHLIGHT DAMAGED AREAS TRY NOT TO BREAK THROUGH GEL COAT.

[Kentish]

Tony, thanks for your post. Just a note of caution, you should never take an unpainted GRP body out in the strong sunlight and wet weather since it will re-shape and start to cure each time it reaches a new higher temperature. There are also many surface pores in gelcoat which water and dirt will ingress. Not a good idea if you like the shape of your car and eventually want paint to stick to it!

Alan,
Many thanks for the detailed instructions, I'll print those off and give instructions to painter before the car goes for spraying. This will be a while yet!

[Vospers-Alan]

Something I forgot to say (tell me to shut up if u already know) but do not use ordinary body filler when smoothing out. We used a special GRP filler which was bloody rock hard to rub down and took ages but the finished article was worth it

[LotusNova]

Nice

[Davi]

with regards to flatting down - best tool I ever invested in (well, made my own copying an original!) was a long bendy sander, cant think of any other way to describe it! It's about 2ft long, 3 or 4 inches wide and made of plastic - not too stiff, bit of flex in it. on that are mounted 2 handles and grips for attaching the paper, on the bottom edge is some thin dense self adhesive foam. You use it two handed like a bow saw, really good for getting a very flat surface without large undulations to give excellent relflection lines.