My Pygmy butt joints went together perfectly, after the first one. On the first one I had bubbles under the Mylar, but I changed my technique and all worked well.

I used a 2-ft. by 8-ft. piece of plywood as my working surface. You have to have a flat and smooth surface like the plywood to make sure the pieces of the butt joint align with no step from one panel to the next. The surface also has to be of reasonable length so you can weigh the panels down in at least two places each. I drew five pencil lines in the middle of the board. The middle line (line 3) was where the butt joint was. The distance between inner pair of lines straddling the joint line (lines 2 and 4) was the width of the fiberglass tape. The distance between the outer pair of lines (lines 1 and 5) was the width of the Mylar strip.

Cover the joint area of the board with plastic food wrap, taped down, taut and smooth. Put the panels you are joining on the board with the butt joint on the butt joint line (#3). Put two bricks on each piece of panel. Check that butt joint is still tight and perfectly aligned with the middle line. Cut a Mylar piece at least two inches longer than the width of the panels. Cut a piece of fiberglass about two inches longer than the width of the panels. Cut 4 pieces of blue tape and place within easy reach. You also need a chunk of 2x4 that is longer than the panels are wide and a couple of bricks.

Spread a generous layer of epoxy on panels out to about 1/2 inch beyond the width of the Mylar (lines 1 and 5). Wait a few minutes for the wood to soak up epoxy and then put the fiberglass tape down between lines 2 and 4. Hold the tape in a loop as you put it down so that it contacts the epoxy in the center; then as the rest of the tape is lowered onto the epoxy it doesn’t trap air underneath it. Tape the ends of the fiberglass tape to the board; this keeps the fiberglass from moving out of position. Let tape soak and then put generous coating of epoxy on the tape and over the edges of the tape. Add epoxy as needed to wet out the tape and have excess on top of the tape.

Now bend the Mylar lengthwise in your hand until it forms a U-shape. Put the center of the U down on top of the center of the fiberglass tape and slowly unroll it in both directions. Any bubbles are forced out ahead of the curve of the Mylar. Now tape the Mylar down just as you did the fiberglass tape. This keeps the Mylar from moving around.

Now put the 2x4 down on the Mylar and then put one or two bricks on top of the 2x4. I put one brick on narrow panels and two on wide panels.

Wait until the epoxy starts to set up before you try to clean up the excess that has squeezed out under the edges of the Mylar. 24 hours later (System 3) the joint is perfect and you can flip the panels over to do the other side. When you do the other side you can put a couple of pencil marks on the board where the panel edges are. Then use these marks to check that the mirror image panel is the same shape before you make its butt joint.