Has anyone tried using old row covers to give tomatoes shade to prevent sunburn when they lose their leaves? It seemed to work well for me and the plants seemed to be revitalized.
Yes, I've used them in the past, though not so much to shield fruit after plants have lost their leaves. I've merely used them when the temperatures start staying consistently hot---say 95 degrees or higher---in order to keep the plants and fruit cooler and to reduce the stress. I actually started out using old bedsheets, and then later on I used floating row cover, and then after that I moved on up to shade cloth.
About 6 or 8 years ago I bought shade cloth and put it over both tomato and pepper plants and they stayed a lot happier and produced more fruit in July and August than they usually do. The shade cloth I currently use in the garden is a 40% shade cloth and the one I use on the green house is a 50%. For me, shade cloth works better than row cover because it is stronger and more sturdy and holds up better to strong wind than most row covers do. However, I was happy with row cover when I used it and I was happy with plain old bed sheets when I used them. It is just that when they got old and tattered and started getting holey, I moved up each time to a stronger type of material so that it would last longer.
I think the main way that shading helps is it reduces the temperatures the plants are exposed to which, in turn, reduces their stress. If you put identical thermometers on the ground underneath the shade cloth/row cover and out in the open in full sun, you'll see how much of a difference it makes.
You also can use row cover or shade cloth to save melons or winter squash if they lose too much of the coverage they get from the plant foliage. I always suspend the material a foot or two above the top of the plants I'm shading because if you don't do that, sometimes too much heat can build up beneath the cover and can cause the plants to overheat and wilt.
In winter or early spring, I sometimes let row cover just float directly on top of the plants especially if I only am using it overnight for freeze or frost protection. In late spring through mid-autumn, though, I erect a framework to raise the cloth higher above the plant, allowing for better air flow. If you don't do that, on a hot, sunny, wind-free day, the plants can roast under the sheets, row covers or shade cloth. With the row cover, there always will be some heat build-up, although the most lightweight row covers don't let too much heat build up. I have some extra heavy duty floating row cover that gives 10 degrees or more or freeze protection and it is very thick and heavy. If I leave it floating directly on plants, once the temperatures are in the 80s, the plants can roast and be severely damaged in just a few hours. So, with that one, I always use hoops to hold it well above the tops of the plants.
You also can use row cover to keep the ground beneath it cooler in order to get cool-season seeds to sprout in the ground in August for the fall garden. Soil in full sun here in August can be too hot for good germination of cool-season seeds. I use row cover (and shade cloth and old bed sheets) in many different ways. In our hot climate, you've got to use every trick in the book to fight the climate and give your plants a little bit of help.
I bought Surround for stinkbugs and can't say it works but the white film does help with sunscald. I had striped blister beetles and leaf diseases and some big tomatoes ended up exposed.