So the problem is that most dyes and paints we use on leather are either water based or alcohol based. So, water/alcohol work as solvents. When you brush on a clear, all the water from the finish leeches into the already dried paint and leather, and the dyes/paints intermix degrading the color or causing runs or thinning. One way to combat this is to spray on several mist coats of your sealer which prevents excess water from penetrating the rest of the project. You can buy aerosols specificically designed for leather applications like saddle lac or wyosheen. These are lacquer based, so they use a different solvent and wont screw with the waterbased dyes and paints. However they are expensive. most sealers or clears you buy at the store are designed to go on hard surfaces, so when they dry, they will crack when you bend the leather.
Recently, I have started using Mop N Glo which works much like acrylic resolene, but its cheaper and easier to get. To spray it on, I mix it 50/50 with water, and use either a prevail sprayer or an air brush to apply several light coats. Has been working well for me. You can even buy hand held sprayers that have a pump, so they work like a prevail sprayer but don't require the cartridge.
mop n glo