Stabilize the roof by putting a ratcheting tie-down around the perimeter.

Start with one crack. Make a few tacks and fill the hole you drilled. Then make v-grooves. For wide cracks,  don’t try to fill it all at once. Lay down a bead on one side first. You can feather the beads together

If the ASA turns brown, you need to back off of the heat.  Don’t get within 3 to 4 inches of the bottom edge. It will shrink away.  More on that later.

Get a nice scraper. It helps to lift one side of the crack so you can make the groove. It is really essential on long cracks as you need some support.

The sharp bends in the roof tend to sag & swell with heat. The flat edge of the scraper works well to hold roof in place while it cools. A stream of compressed air works wonders here.

Taking a few pieces of welding rod across wide cracks helps to stabilizre the crack.  Using heat on the edge makes it pull back and curl. I found that you can solvent weld these tacks by dipping the ends in Methyl Ethyl Ketone (MEK - available in paint stores) until they swell. 

Roughen the roof and wipe it with a little MEK with some welding rod ends dissolved in it. This technique also works well for the little cracks.  I found that a cone shaped burr on a dremmel tool makes a nice groove, but be careful, you can create a big gap quickly!

Getr a small jar and clip a couple of welding rods into 1/8 to 1/4 inch pieces. Soak these in a little MEK to make a putty.  Go easy on the solvent. This may take a half a day to make.

You will make some errors. The roof will sink. You can fill these by solvent welding small pieces to fill the depression, then sand it down later. It takes a long time for the solvent to evaporate. Again, paint a little MEK with dissolved plastic onto the roof to get a good bond. A very small spatula is handy for this work.

You should use the heat and light pressure to feather these solvent welded pieces together and to work them into the roof material. If you see bubbles, stop and come back later.

When you get it filled, sand with 80 to 150 grit as needed to make it fair and smooth. Paint it with your MEK with dissolved plastic. Feather this into the unsanded areas of the roof. This will give a nice sheen to match the original roof. Use hot air, in broad sweeping motion to melt the surface .

You’re done! Congratulations!

The very long crack is stable now.

Watch for small hairline cracks which may develop in a day or two. Use the cone-burr to open these, drilling stop holes at each end and solvent weld them closed. Feather this into the existing material with minimum heat.

The weld is not very visible in sunlight!