It's always a good idea to strain paint before you start your project. It takes a little bit of work to strain your paint but if you allow for this & build the extra time into your project, you'll have done yourself a favor. Here's why:
Some paint stores rotate their stock, meaning the paint cans in the back of the rack eventually get moved to the front of the rack. The paint stock can have the tendency to form small clumps (or lumps) in the actual paint cans, depending on how long the paint cans have been on the rack prior to being sold. I'm talking new cans of paint here so take note. That's what I'm saying. Not only should you strain your old paint, you should also strain your new paint! How many people have told you THAT before?
I have seen this first hand as it happened to me one "night". One of the workers was opening a 5 gallon container & he was ready to start applying it to a wall & he noticed there were clumps in it so he told me about it. We ended up having to strain paint in 10 of the 5 gallon containers (that's 50 gallons of paint). I was not prepared to strain the paint since we were ready to paint the office that evening & all I had time for was to paint all night & finish just before the tenant came to work the next morning (all-nighters are not fun) plus I had asked the paint store to check for this anyway (grumble grumble).
Luckily I just had time to get to a store before they closed so I could purchase a paint strainer bag. I don't know what would've happened if I hadn't gotten to the store before they closed. The job was held up for an hour or so then I had to pay one of the workers to strain all of the paint. We still got the job done on time but I was sweating it out at the end.
I had trusted the paint store to check out the paint containers for lumps prior to mixing & shaking up the containers & alas...this was not relayed to the person who was actually mixing up the paint......say what? I questioned the paint store about this the next day & they said sometimes lumps can form in paint stock even before they purchase it from the manufacturer (or their headquarter main warehouse)....so buyer beware. This means you. You could be "the chosen one" next time. It doesn't happen very often though but I wanted you to be aware of it & be prepared, that's all.
I now always make sure I have paint straining bags with me before I start a project since straining new paint (as well as old paint) just makes sense. I generally use the paint straining bag type you can purchase at any local paint store. They have two sizes: one size fits on gallon containers & the other fits on 5 gallon containers. I would highly suggest that you purchase both sizes as you will eventually use one or the other. The paint straining bags are made of a very fine cheese cloth material & have an elastic edge sewn on the top edge. They really come in handy. Get a few extra too as they're cheap insurance. The straining bags also store nicely so there's no reason not to have them on hand.
I do not recommend using old panty hose since you have to deal with "runs" & "holes" plus it's a little harder to work with to strain the paint for your project. It is possible though, so in a pinch, do what you need to do.
It's a must to strain old paint so don't even think twice to "strain that paint". I'm talking about straining any old paint even if it's only a few months old since you don't usually see the lumps or mini clumps until you have mixed up the paint & begin pouring it in a paint tray ready to paint your project. That's not when you want to see these "suckers" magically appear. And don't think for a moment you can kind of "miss" the clumps with your paint roller because let me tell you......those rollers are like "magnets". They pick up everything then spread these little guys all over your wall before you can say huckleberry hound! I know. I've learned by experience & I want you to learn from my "now known knowledge".
In a nutshell, strain old paint, strain new paint, if it looks like paint & smells like paint then "strain that paint". You've learned alot here...........I hope.
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