gelcoat problem

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#1

I’m working on a project car of a friend of mine where several parts will be cf (hood, right-left door, rear door), right now I’m in the process of the mold making. until now everything was great but today while i was gelcoating (polyester) the back side of the hood i had some shrinkage (alligator skin), i’m pretty sure that this happen due to the different gelcoats i used. let me explain, in the first layer i brushed a red, low viscosity gelcoat (i’m guessing it was for spray gun) for the majority of the surface but because i didn’t have enough quantity i also used an orange, thicker viscosity. then 3 hours later i brushed the second layer of gelcoat, this time i used only orange, during the whole process i had the mold outside into direct sunlight and 20C temperature, but a little later i saw that in the places that i overlapped the red gelcoat it started shrinking :(. i had no knowledge of the difference in the gelcoats until i used them and unfortunately i never thought to check them before i start the process because it bought them as the same kind of materials.
now my question is: can i laminate over the gelcoat as it is (i don’t mind if the mold has surface issues because my thought is to sand-bondo-primer-clearcoat it afterward) or then only way is to remove it and start over again? if i have to remove it what’s the proper way to do it without messing with the upper half mold which is already finished?


i tweaked the close view pic so it would be more obvious the problem

#2

I don’t think it is mixing the gelcoats, I think it is too thin gelcoat on the first layer. The second layer attacks the first and you end up with gators. You can continue to build the mold if you want but you will have to repair the defects. Personally, I would start over.

#3

How would you remove that gel coat without damaging the plug or at least minimising the damage?

sent from GT-I8190

#4

Laminate over it, then pull it off.

Alligatoring is caused by undercure of the first layer. This can be due to:

bad mixing
low temperature
low on catalyst
thin layer
not uniform layer (streaks)
humidity
too fast overcoat time

#5

As Herman say this happen because of the mentioned problems. Usually this happen especially if the catalyst is not the right amount.
Use syringe with ml everytime you make the mixing and not with the eye!!!

#6

In this case I suspect the spray gelcoat has been laid too thin, as it is low viscosity. It also contains acetone, so it might be more porous when brushed.

#7

it was laid to thin but also the molds of the rear door and the upper side of the hood which i have finished the first layer was thin and the second was for full coverage, i’m thinking that because the orange gelcoat cured faster didn’t allow the acetone of the red to evaporate.
since i have to laminate it in order to remove it i will do multiple layers and if i see that the damage is too major i will re do it otherwise i will repair this one.

p.s. i’m always weighting the materials

i appreciate everyone’s advice

#8

Forget repairing this. Pull it out after one light layer. The time consumed is much less, and in reworking you lose the same amount of even more in materials (filler, sandpaper) than the lost gelcoat + laminate.

#9

Why do you think you’ll damage the plug?

A million years ago when I was first learning composites, the standard treatment for a new mold was to spray a heavy coat of gel and then blow it off (we called it a peel coat). It was supposed to condition the mold surface. It usually came off in bits and pieces and we (the grunts) would have to clean it all up while the old guys took a break. Pain in the backside getting all the little chips up. I suppose you could do that or, like others have suggested, a single layer of glass and then pull it off.

#10

so i followed your advice and i didn’t laminate it as a normal mold but i did one layer of csm 225gr and today i took it off, i believe that the time to repair the damage would be probably the same as the one i will spent preparing the plug again BUT the positive thing of doing another mold is that i will improve the way i closed the gaps between the metal sheets. i closed them in such way (using plasticine) i had 90 degrees in these spots and that would be a time consuming nightmare when the time to place the cf comes, now i will give some contour and i hope it will make my life easier later on.

i’m posting some pics so it could be a complete thread in case someone else bumps into it.


once again i appreciate everyone’s advice

#11

I would go back a step and question why you are making a mold of the back side of the hood at all.
The structure you are molding was designed to reinforce the steel shape. That shape is irrelevant to a composite hood. Just mold the top side of the hood and place reinforcement in the mold when creating a part or make a bottom half mold that has simpler reinforcement suitable for a composite hood.
You are correct that molding that shape is going to be very difficult, but it’s unnecessary.

#12

Golden advice there… That’s what I thought when I saw it. You can use core, stiffeners, pieces of foam with some shape… whatever to add the extra rigidity and not add weight. That’s what airplanes do all the time and the forces a jet will see on a fairing is way higher than just about any car

#13

i will add core material (either 3d core or soric xf) but the car owner wants an “oem” looking end result and i think that this backplate will be an extra safety factor against possible warping. also, making a different reinforcement backplate would be much more time consuming job rather than coping the existing one.

#14

You don’t need a backing plate at all if you are using a core and stringers for reinforcement, just lay into the mold for the top. Some people insist on doing in top and bottom halves depending on the process they are using(chopper gun) but the bottom would still be very simple.
I’ve never seen anyone mold the back side steel reinforcement. It just doesn’t have any relationship to a composite product. If the customer insists on having it for looks, you better be telling the customer that they are dramatically increasing the cost of the hood because you are going to spend a ton of time getting a workable mold out of it. No reason to do it besides cosmetic.

#15

stealth could you please give me a link to see info’s for the reinforcement stringers you are talking about

#16

I tend to disagree on this point. At least as an across the board way of construction. When customers prefer a stock look or even like to re-use their hood insulation mount points it is required.

Also, if engineered to optimize the overall combination of cloth choices,(e.g.: uni-directional,bi-directional, thin cores and graphites etc. one can accomplish the same light weight as a ‘SKIN’ yet be much stronger in the final outcome.

I do all of my trunks,hatches and hoods this way. It does require more design time and mold considerations…but I enjoy that process.

#17

Stringer is just a term for a reinforcing beam molded into the composite part, can be made of all sorts of materials, but I guess the traditional “stringer” coming from my usage would be balsa or other wood as in surfboards or boats.
For the few hoods I’ve made I used divinicell strips wrapped in cloth laid into the laminate . I’ve seen some hoods made with no reinforcement beyond the flat laminate stack.

My terminology may be suspect, I’m just learning molding composites myself.

#18

Interesting. Do you make a visually identical reinforcement part or do you modify it to be easier molding within the same dimensions? Molded as two seperate halves or a two part mold? I have found these types of panels very difficult to duplicate identically.

Not sure what you mean about the skin, but I was thinking that reinforcement designed with composites in mind instead of steel can be lighter and simpler as well as stronger.

#19

Most often, if the substructure’s original basic form will suit, I’ll use the bulk design modified for better draft and removal of zones that I won’t need. In some instances, I’ll make it a full skin as well and secondarily bond both inner and outer with epoxy adhesives where appropriate.

#20

Thanks for the details
But it seems then that the OP needs to modify the shape of the reinforcement to be able to mold it, yes?