From all my experience with Gel Coat and aged fiberglass there is nothing that can be used as a reasonable bonding agent to work with the old gel coat. Everyone replies correctly in that regard. The material has simple been robbed of the structural binder through exposure to the elements. Adding new goop to it is not going to increase the decay of the old stuff and it won't adhere to the old stuff any way.
So yes, sadly the old stuff must be removed in order to make a more sound body. However, you CANNOT get a reliable lofting from photos and no one will make the body correctly by eyeballing it. All surfacing is interpretive. The hand work of the original builder is a fingerprint of his capability and artistry. * The best way to preserve the origins of the car and properly tribute the original fabrication and surface development is to create a record mold. This might sound exhaustive, however it is quite simple and worth its weight in gold as a means of preservation.
To begin with, the entire body must be cleaned and lightly brushed so that contrasting 1" tape can be applied at every 8" section where decay has presented. If this is the entire body, then the entire body might need to be taped across the body from side to side to create section lines (essentially slicing a loaf of bread). Take LOTS of photos these will be your valuable records.
Then in the areas where new body material will need to be created, create a mold of the area. These molds will serve as your "body contour splines". They only need to be as wide as the general area.  Number them consistent with the area on the car from front to back and label the sections in the photos. SAVE these record molds and offer them to the next guy who will be restoring a car like this.
Essentially this is a reverse molding process that will yield a properly finished car body exactly as the designer and builder had intended. Yes, it does add to the development/restoration process but it is worth it and no one will ever argue that you have "recreated" a car with a modern interpretation - because you will have patterned ORIGINAL contours. All the correct and original contours will be preserved and usable through the template splines for future collectors to employ.  If you require a drawing to show this process I can sketch it up and send it. Also I can further detail the process if needed.

Raffi, Sorry if I keep looking for more answers. I just figure that that's more research, and more is better (up to a point). I contacted this guy at West Systems since I figured he worked for an epoxy company and would know the limits of their products (which are well considered). I think this is pretty much the same answer as Dave really. In the end you grind off all of that gel coat since you can't trust it to not continue to craze and crack. I guess if you work in small enough areas, you probably won't lose a lot of body lines.
Am continuing on the Weber/ Flathead trail, but haven't heard anything back from the Flathead Dr. Mocked up a portion of intake last weekend with rigid urethane, and am thinking I'll make up the whole thing in styling clay (which I have a lot of). This isn't as nice to form, but should be sturdy enough for a machinist to mill a block of aluminum from.  Regards, Bill

Dear Bill,
I would tend to agree with the boat restorer you talked to. Gel coat isn't structural and is usually applied at about 20 - 25 mils. Given the current condition of the car, if it were mine, I wouldn't trust that filling the missing or bad parts with epoxy would keep the rest of it from delaminating and cracking. Simply put - the gel coat is beyond repair.  I recommend carefully removing the gel coat so as to not damage any of the fiberglass. If there is detail that is in the gel coat only, take enough pictures so the detail can be re-created. Coat the entire body laminate with several coats of epoxy (West System 105/206), wet sand to prep for painting and then paint with a quality 2 part urethane or epoxy paint. I would not recommend you re-gel coat, unless there are historical/authenticity reasons to do so.

Mr. Niederer, I wonder who I might contact that could advise me on repairing a 1950s fiberglass car body called an Allied. This is a fairly unusual item of  which less than 10 survive. My own car was constructed by an Indy 500 race car builder Jud Phillips, but never completed. For this reason, while the gel coat was painted with some sort of white paint, the vehicle has sat outside for most of its life. The vertical parts of the body still have white paint on them, and appear to be fairly smooth.  However, the horizontal surfaces range from small crazed areas to cracking.  More than one body shop has refused to even consider restoration (since most automobile restorers work in metal and don't know what to do with fiberglass). I've had better luck talking with boat people, and have a yacht restorer interested in doing the project. His inclination is to grind the existing gel coat back to the cloth, and re-resin it, but I'm wondering if there are epoxies made which could be used instead to  fill and coat the cracks, stabilize the existing resin, would adhere  to both old resin and new body filler, and be able to of course handle  automobile primers and paints. My reasoning for not removing all of that old material is the concern that I might lose the body definitions in the process.

Question from Bill:

Now I'm guessing you've been through this a bunch of times, but I'm a bit anxious about the body on this Allied. I feel pretty good about where I'm going on the mechanicals, and for the most part (except for the serious welding), I feel like I can and am doing a lot of this project on my own. That's important to me from both an accomplishment standpoint as well as an economic one (we can't all be orthopedic surgeons).  This said, through a friend of mine, I got a man well versed in boats to look at the body a week or so back. He's very well recommended by my friend, and I like him. Originally before seeing the car (only seeing the pictures), he had suggested an epoxy material from Smith & Co. You brush it on; it fills in the cracks and remains as solid or flexible as the original surface. Lots of boat folks use this for repairing rotten wood even. But after he saw the body, he said that he thought the gel coat would have to be sanded down to the mat to get rid of the enormous amount of crazing. As we've talked about, the body was probably never sheltered, and so while the vertical areas are still painted white, and are relatively smooth except for a few imperfections, the fenders, hood (which will be replaced with aluminum), roof all the way  to the tail, has a lot of crazing. The fenders have quite a few pinholes (which of course go all the way to the fabric). The biggest damage is the big dips in the roof. I know these can be pressed back to shape, and of course reinforced at that ceiling with additional conduit. Another method would be to cut them out and just simply re-glass that whole are from scratch.  While I believe the methods Roy and the other makers were using were the same as the fiberglass boats of that time, I'm not sure if I use those techniques and products whether I'll be disappointed. Car bodies face different stresses than boats, and that may become a problem. Who is the best authority on all of this? Are there other people who have restored bodies like the condition of mine? What works and what doesn't from a long term perspective (I don't want to refinish that body every five years).  While I'm not planning on painting this car myself (hey, I know my limits), I want to deliver to my painter a body that won't require much rework, hasn't cost me a fortune (can you imagine hunting down every little crack in all of this crazing?), and will stay stable for years to come with good maintenance.  Suggestions, Opinions, Ideas?

Answer from Dave Perry:

It's a gamble, commenting on the body condition described, without actually seeing it. However, it sounds pretty much like many of the 1950's bodies we have here.
First I would correct the roof shape, and probably by using 3/8 Airex foam sheet glassed in place. " pop" the roof, lay a sheet of airex on top and heat with a heat gun until it reasonably conforms. Turn the body over, put a layer of wet mat then the airex, with a weight to hold the correct roof shape. after it cures, a couple of layers of light mat over the airex should give a nice looking composite roof that will retain it's shape. A little more expensive, but this will look a lot better than conduit bows. If the gelcoat is thick and chipping or flaking, it should be sanded off with a DA sander. Also , any areas of spider cracking as well. Doing ANYTHING over these will , at some time, lead to a redo. It is important to look at EACH area and determine WHY it cracked. Some may be a result of storage or moving. If not, consider whether the area is stessed and needs reinforcement, or is a result of flexing and needs stiffening. In some areas, it may be a good idea
to just put 1 or 2 layers of "veil " , on the sanded surface, then sanding to approximate shape. Airex is great along the rockers. It is easy to shape, and is a permanent composite that finishes nicely.  After the basic repairs are made and the body its sanded with 120 paper, wet it down and see if there are any spiders that need attention. As I said, in small areas, we sand them out, put some veil and rough sand to shape. The body can have NO dust in any of the pinholes, etc, so blow it off TWICE. Make sure you have filters on the air hir, as compressor oil in the air is deadly.  We use Duratec sanding polyester , following the directions on their website http://www.duratec1.com/ProductList.html#DLBA especially the 707-002. This is an in-mold repair material, sprays and drys like 2-part paint, sands nicely, and accepts automotive primers and paints. FOLLOW THEIR online DIRECTIONS .  Then , we begin block sanding. the highs will show. It can be recoated, skim coated, or most any automotive technique applied. When applied correctly, it will seal the fiberglas as well or better than anything I know of at this time, AND it sands easily. By itself, it can be compounded , waxed, and buffed to a high gloss. Of course we don't do that, unless we are taking a mold from it.

Two errors I generally see by beginners.... trying to laminate cloth directly to existing fiberglass. Using a layer of mat, then the cloth, gives better adhesion, the coarser weave, the more unlikely it is to adhere permanently. ALSO , many fiberglass resins that are used have paraffin ( wax) in them, which floats to the surface when curing. Trying to laminate onto them, whether old or new , without removing the wax, will not produce long lasting results. Be sure to check the resin you are using to make sure it is not waxed. If it is, you MUST clean it before doing anything. Buy resin that has no wax is the best soloution. More resin, primers, and paints will NOT stick to a repair that has wax.
The second is not taking the extra time to be sure there is NO dust in any pores or remaining small voids and cracks. It will defeat the use of the surfacing sealer.
I didn't say so, but we wet the down side of the Airex foam to improve adhesion, before putting it in place cover the wet mat.
Also, for repairs, using the finest weave you can locate will give the most strength against tears, etc. It also is easiest to conform to multiple compound shapes.
Epoxies are most difficult for beginners. For car bodies, I only use them if I MUST bond to metal, otherwise we have used polyester resins for over 50 years.

Dave Perry
OldSchool Restorations of North Alabama
www.race-cardrivers.com

I noticed that Bill had added a note on the link about styling clay and working with that for new forms. The best clay is Chavant Industries clay. This series of automotive styling clay is the J-525 which is always pliable and can be heated to almost liquid texture for quick applications, cooled and then splined or scraped for overall contouring.
Takes a lot to do a full sized car this way but it is the best way to add and subtract material. When cool, it can also be primed and sanded and used as a master buck for a splash mold.
Raffi