Rust penetrant manufacturers are constantly working to improve their products. The latest formulations work better than previous brews. So if you’re still using a 10-year-old can of penetrant, you might want to upgrade to the latest formulas from Liquid Wrench or the Specialist Rust Release Penetrant from WD-40.
As great as those new products are, occasionally you can get by with a home brew—a 50/50 mixture of acetone and automatic transmission fluid. It has a strong smell and is far more flammable than commercial products, so make sure you use it outdoors and away from flames. It doesn’t work as fast as the commercial products, but if you need rust penetrant in a hurry, it will work.