Warning! I used scanned photos on this page. While I have them heavily compressed, they will take a few minutes to load.
Second Warning: While the techniques I used here were not very difficult, this was not a simple repair. If you have no experience with fiberglass you might want to have this done for you. Start with a simple hole and work your way up. This is especially true if you are hoping to achieve a decent looking repair and still maintain the structural integrity of the boat.
OOPS!!!
This is what happens when your boat blows off a dock during a storm and spends a few hours drifting around the harbor banging off various obstacles. In this case the concrete walls around the harbor seem to have had a magnetic attraction for my boat.
This first shot shows the starboard quarter of my Vanguard 15. This is primarily cosmetic damage extending only through the gelcoat. The middle photo shows the port quarter rail. This is actually worse then it looks. The structural effects of the rail hitting something very hard go far beyond the visible area. The last shot shows the port quarter lower down the transom. Here there is abrasion of the gelcoat extending though the outer layer of glass. (The wide scrape is where I used a grinder to take off more of the gel coat so I could see if there were any hidden impact cracks or areas of delamination.) This damage is not to bad, but will require some fiberglass as well as the cosmetic repair.
Step 1. Assessing Damage
One of the hardest thing for a DYI to do is figure out how far the damage actually goes. Is it just the gel coat? Is it a hole or an almost hole as shown above? Does it involve stress points, ie ribs, hardware mounting points, etc. In most cases people tend to under do the structural repairs and then over do the cosmetic. When looking at a repair it is best to keep in mind the following goals in order:
1) The area being repaired must end up structurally sound.
2) It must be functional. ie if you are repairing a hull, any surface should remain fair.
3) It should look good when it is finished. Original if possible, but at least neat and workman like.
Step 2. Prepping the Area
In the case of the above damage, I split the repair into 3 basic parts: the hull/transom; the aft corner of the deck and the cosmetic. This is the stage where we find out just how bad the damage is. I guarantee you that it will always look worse before it gets better.
I started by allowing the hull to dry out for several days in a heated garage. During this time I removed the hardware from the areas that would need repair. I then cleaned the areas to be repaired with acetone. This is done to remove grease and dirt that might get ground into the fiberglass and hinder lamination of the new sections. These are the same basic steps I use before even starting a repair. How long the boat needs to dry out is determined by how long the damaged area was exposed to moisture.
Step 3. It's going to look worse.
After drying I started by removing and cleaning up the damaged areas. I slowly began to remove gelcoat from the affected areas with a 4" grinder. I did this to see how far any impact damage might extend. Keep grinding away untill you have removed enough gelcoat to expose any cracks or areas of delamination that extend from the damaged area. Delaminated areas will often have a lighter colour. When put under pressure they will crack and give. Glass fibers will shatter and shear under impact and look kind of like cracked glass. (PS. Make sure you where both a dust mask and safety glasses when grinding or cutting the fiberglass!!!!)
Here you can see the 1 1/2" area that has been ground away along the transom. The area has been tapered into the existing glass and is deep enough to allow for 2 layers of glass to be relaminated.
Rail is Ready for Filling 
Here is the end result of grinding my way back to undamaged glass on the rail. I followed all the cracks and slowly ground away at them until I was sure that the damaged areas were removed. Look for hairline cracks in the gelcoat and slowly grind the cracked area away. Often you will find the glass structure underneath has been quite heavily damaged. As with the transom I have feathered the edge of the repair into the undamaged material so that I am laminating back to solid glass work. Cracks or no cracks you must grind untill the full extent of the damaged glass has been exposed. Just feathering the area around the hole and slapping glass back in does not do any good. This is the most common mistake made by do-it-yourselfers. You must be laminating to structurally sound material.
Finish this part of the project by cleaning dust away from the areas to be repaired and wiping those areas down with acetone.
Step 4: Reconstruction
When doing repairs like this I tend to work the boat in stages ie all the grinding together - all the laminating etc. It is often much easier for most people to break a project like this down into sections and get one section mostly done before working on the next. Do whatever makes it easiest for you.
Claw Glass Filler on Rail 
I began my reconstruction by applying some "claw glass" (fiberglass resin with chopped strand filler) to both the back quarter rail and transome corner. I used it to fill some of the deeper gouges through the glass where I could not justify grinding any further. When the filler has set, I ground it back down with both a grinder and sander so I would have a nice smoothe area to relaminate to. Despite the way it looks from the picture above, when ground the filler was no more than a sixteenth of an inch thick. Clean away the dust and wipe the area down with acetone.
This is a good time to talk about cloth. Unless I have not other option I always use layers of fiberglass cloth with layers of chopped strand mat in alternating layers. The mat helps bond the stronger layers of glass cloth both to the boat and each other. I usually try and grind away enough material for at least one layer of mat, one of cloth and another 16th inch for gelcoat or filler. The next step was to flip the boat over and place the 2 layers of glass over the edge of the transom where I had ground it down. Due to the length of the strips of cloth I decided to wet the cloth out with resin in place. I started by painting a coating of resin onto the area I was repairing. I then applied the 4" by 48" piece of mat and coated it with resin. Do not use a lot of resin. Just enough to make the mat turn clear and mold to the edge of the boat. (Note for all these repairs the cloth is cut slightly bigger than the repaired area.) I then applied the layer of cloth right over the still wet mat again applying only enough resin to "wet out" the cloth. Use a paint brush to apply the resin. Work the cloth down flat and use the brush to push out any air bubbles. Use as little resin as you can get away with. Extra resin only weakens the laminate.
Side view of the transom corner repair. 
The corner of the rail is more complicated. Reconstructing the overhang meant using a form so I had somthing other than air to laminate against. The first drawing below shows the form for the rail. I used wood shims cut to shape and waxed to stop the glass from adhering to them. I held the shims in place with duct tape. Dashed line in the drawing shows where the rail needs to be rebuilt.
Shims used for rail form. 
I started by cutting the pieces of mat and cloth I would need. The diagram below shows the shape and order of the layers. Notice that I rotated the direction of the cloth weave by 45 degrees.
Alternating Layers. Layer 4 is closest to the hull. 
Since the pieces of cloth here were small, I decided to wet them out and build up the laminate before setting it in place. You can do this easily eonough when working with 4 layers or less. It makes it much easier to control the amount of resin used and keep air bubbles from forming.
I started by taking a scrap of cardboard and painting an area slightly larger than the cloth with resin. I then started with a layer of cloth (The layer that would be on the outside.) and placed it in the center of the cardboard. I coated it with enough resin to turn it clear. I then placed the next layer (mat) on top of the cloth and repeated the wet out procedure. This was repeated for the last 2 layers. (Remember you are typically building up the laminate in reverse. Dry fit it and lay it down in the proper orientation. Also try and start with a piece of cloth as the first layer rather than mat. It makes it much easier to get the wetted out pieces off the cardboard.)
Building Up the Laminate 
Once I had the pieces wetted out and laminated together I coated the corner of the boat with a light coat of resin and laminated the pieces into the final repair. Use the brush to press them into place and push out any air bubbles. If you need to cut the cloth at the corner go ahead. (See diagrams below)
Let the fiberglass cure. If you are not comfortable working with 4 layers at once, use 2. Let the first 2 layers cure, wipe the area with acetone, sand lightly, wipe with acetone again and put on the next 2 layers.
As a last step I pulled the froms out, ground the repair to shape and added a few small pieces of cloth to the back side.
Step 5. Finishing
If you did the repair with polyester resin, there are 2 choices available when finishing. The easiest and quickest is to finish fairing the hull with marine filler and then paint the filler to match the hull. The second option, is to fair the hull with filler, Sand it back off again so it is a little low, and then sparay it with gelcoat which will then need to be sanded through the grits and finished.
For this repair I used both methods.
For the hull and transom edge I started by wiping the area with acetone and sanding it off lightly. I then faired the repair with putty (formula 27). I finished the putty down to 320 grit wet and then masked and painted the repair with 4 coats of white laquer.
On the transom corner I faired the repair and knocked it back down to about 1/16" below level. I then sparayed the gelcoat on using a "pre-valve" sprayer. For this repair I had to deal with 2 colours of gelcoat. I did this by making off sections as you would for paint. If you are going to spray gelcoat, it can be thinned with acetone or styrene. Use as little thinner as you can and still get it to spray. Gelcoat is not like paint. No matter what you do it will leave an "orange peel" type finish tthat will have to be sanded, so keep the thinner to a minimum. Also if you buy gelcoat from the factory you will need to get some sufacing agent to mix with it. Gelcoat is an anaerobic resin and will not set properly in the open air. The surfacing agent is a wax that will rise to the surface and allow it to cure properly. When cured, whipe the gelcoat off with acetone and wet sand it down to finish it. You will have to work your way through the grits starting with 220 or 320 and finishing with 1500. After several days you can buff the gelcoat to a shine iwth some polishing compound.
A couple of notes:
Gelcoat is not a good filler. Fair the area with putty and then oversparay them with gelcoat.
It is almost impossible to get gelcoat to match the original colour unless the boat is fairly new.
If you decide to use epoxy resin, you CAN NOT PUT GELCOAT OVER IT!!!!!! You can put epoxy resin over polyester but NOT the other way around, it will not adhere properly.
DAN, I DON'T UNDERSTAND YOUR DIRECTIONS!
Email me and I will be happy to help.