It probably isn't a bad idea running some vinegar through all these panels as they are fixed up.
Also, I was able to stop by a home improvement store today. They did have all copper sweat-on unions there. The nut on the union was either cast copper or bronze. The unions cost about $11 each! Not cheap, but only TWO would be needed per panel.
Each panel would get ONE-HALF of the union on the right side of the pipe, and the other half would go on the left. When all the panels are laid out side-by-side, each one will form a complete union. Just screw together, and you have a solar hot water system!
It has been the sunniest week of the late winter so far! Of course this weekend, when we can actually RUN the panel, I am sure it will be cloudy the whole time.
In our climate, these panels will need to be in a closed-loop filled with glycol. For testing, water will work great.