Background: Canasta is a 1957 Shasta Deluxe that may or may not have had wings in the beginning. It appears she didn't, but hard to tell. (late 57s may have come with them) These wings do not seem factory, but are nicely done and are close to the correct size. They mount flush to the body rather than with spacers.
There were no backers for the wing screws. (yes, I added some) Note the water stain at the screw hole:
And lots of extra holes. It's obvious the wings were mounted at least twice.:
The J rail is cut to accommodate flush mounted wings. Putty was used in prep for mounting. The wings had multiple holes, so waterproofing tape was added. Putty was doubled over at the top edge:The small hole is a drain for the wing. They had styrofoam inside, no wood.
The top edge was sealed. As you can see, the putty is very important on a flush mount wing. The putty was smoothed and "poked" to form a good seal.
I don't think using the spacers will be that different. It's a matter of good BACKERS to support the weight of the wing, plus sturdy screws with sheer strength. Of course you wouldn't need to seal with putty, the spaced wings are meant to drain. They all too often collect debris and don't drain...
I'm not sure which is better. Hopefully, others will add to this thread with their ideas and experience.
Shasta did it all right. I've not had one yet with any backing...
The putty is sold at most RV stores; it's called "butyl putty tape". (not real butyl tape; identified by slick paper) The old standard stuff is tannish, while the other is white. I recently had to order the white because the new service manager likes the old standard type. Either will work.