Overmixing crepe batter develops too much gluten, resulting in tough, rubbery crepes instead of light and delicate ones. A few lumps are perfectly acceptable and even desirable.
Crepe batter is a simple mixture of flour, eggs, milk (or water), and melted butter. The key to perfect crepes lies in the delicate balance of ingredients and minimal mixing. Flour contains gluten, a protein that develops when mixed with liquid. The more you mix, the more gluten develops. While gluten is essential for the structure of bread, it's the enemy of tender crepes. Overmixing leads to a batter that's too elastic, resulting in crepes that are chewy and lack the desired thin, lacy texture. Aim for a batter that's just combined, even if it has a few small lumps. These lumps will dissolve as the batter rests. Resting the batter for at least 30 minutes allows the gluten to relax, further contributing to a tender final product.
Use a blender for a quick and lump-free batter, but be careful not to over-blend. Pulse the ingredients until just combined, then immediately transfer the batter to a bowl to rest. Avoid the temptation to blend it smooth.