There are some small bubbles but I generally accept the way it looks. I will try to flatten them but do not plan to cut or inject paint inside with a needle so not to break the surface of canvas.
As I planned, I did not use a glue at all. Just a paint, which gets naturally sticky when dries. Sometimes when I left hair from a brush on painted surface I could not remove it after it dried without breaking a layer of paint. The same way I glued canvas. As there was painted plywood underneath it was watertight itself. Applying canvas and covering it with another layers of paint gives enough protection against water, I think.
I designed the final version of a painting scheme. On the picture attached it is just a semi- transparent layer of gray patch object applied in Inkscape, not a real shape painted on a trailer. But I will paint the same shape on a real trailer this week, if it does not rain. What do you think about how it looks?