The correct way to seal any lace front units, and this includes Closures, Frontal, Lace Front Wigs and Full Lace Wigs, is to comb all the hair going forward, pin the unit down as shown in the photo below and proceed with whatever you're using to seal you units. This prevents the hair from flowing in one direction only. If you seal your units going backward, it will ultimately flow that way permanently. By sealing the knots in the forward direction, this guarantees that your units will flow in every direction.
In the photo above, I use Knot My Problem, Clear Acrylic Sealer and Pump it Up after I've finished ventilating. None of these however are very permanent, however they will provide a strong hold until you wash your unit, then you will have to re-apply all three of these items again. I don't wash my units but once a month so it's not really that time consuming to re-apply these items.
In the video below, I use two products to seal my units for a permanent hold. They are Knot My Problem and EasyTab. This way I do not experience any shedding. I also seal every closure that I make for clients this way. Do not under any circumstances apply the Easytab directly to your lace unit. Always use your finger or a sponge to apply it. You will damage your unit tremendously if you apply it directly to the lace..... you have been warned!!