Straining is a topic that we talk a lot about in our Prep to Finish paint training program (preptofinish.com). Conventional wisdom and common sense dictate that straining is a great way to remove contaminants from product while it is still in liquid form. This is certainly true, but there are other, more subtle considerations that make straining a good idea.
Specifically, building the habit of straining ALL product every time you load your cup gun is cheap insurance at a deeper level. As the product world transitions more into waterborne platforms, the practice of shaking a can of product is not recommended. Shaking a waterborne tends to bubble it up right in the can, and even after the bubbles appear to have settled out, they often still transfer through the gun and into the finish.
This makes sense, because even in a low pressure system, there is still pressure and force driving the liquid through the needle. You may not be able to see large bubbles in the cup, but there can easily still be tiny ones inside the liquid form product, that will pass through your gun tip, and lay down with the finish as micro bubbles. This creates backwards motion, because usually we will have to sand the dried micro bubbles smooth to remove the nubs, and do another round of spraying.
I recommend a soft stir and then a full strain of all new product. Using a fine mesh cone style strainer, the product actually gets a “combed” effect through the strainer, so that it enters your cup with ideal consistency and viscosity.
Lately, we have rediscovered the convenience of using a filter stand when preparing product for HVLP spraying. If you don’t have a strainer stand, I highly recommend that you pick one up, as it makes it easier and more convenient to ritualize the straining habit. You don’t have to stand there and hold the cup and can. You can be finalizing your tack wipe on the piece to be sprayed, so there is a bonus efficiency gain through this simple implementation. Simple multi-tasking at its best.