Most callbacks on a crack injection job don't come from bad technique. They come from the wrong tube on the pump. A resin that's too thin runs straight through the wall, or leaves nothing behind but a smear on the crack face. A resin that's too thick never gets into a hairline at all, so you seal the opening and the crack keeps working behind it.

Viscosity is the one variable you choose before you climb the ladder. Here's how to match it to the crack in front of you.

Read the crack before you pick a tube

Three questions decide the whole job:

• How wide is it? Measure at the widest point, not the average. Cracks taper, and the widest section is where the resin has to do its work.
• Is it moving or dormant? A dormant crack can be filled. A working crack is a structural conversation, and injection alone won't fix the cause. Get an engineer involved before you sell the repair.
• Is it wet or dry right now? Moisture decides which chemistry you reach for, more than width does.

Answer those three honestly and the resin choice is usually obvious. Skip them and you're guessing with a pump.

Hairline and fine defects: thin resin, injected slow

Tight cracks are a viscosity problem. Thick material bridges the opening instead of entering it, so the crack stays open behind a cosmetic plug.

This is where a super low viscosity urethane does the work. Our #700 Super Low Viscosity Urethane is built for penetrating and sealing fine concrete cracks and hairline defects, and it's the tube to reach for when the opening is too tight to accept anything with body.

Two habits make thin resin perform:

• Inject slow and low. Thin resin is unforgiving of pressure. Push it too hard and you'll run it through the wall and into the soil, or blow the surface seal off the face.
• Seal the face first. Standard practice on tight cracks is to surface seal the crack face, install your ports at the spacing the resin manufacturer specifies, and then inject. The seal holds the resin where it belongs instead of letting it drain out over the wall.

Wider cracks: you need body, not just reach

A wide, dormant crack is the opposite problem. Thin resin wets the faces and runs out. It coats the crack walls and leaves the void in the middle of the crack empty, which is exactly the void you were hired to fill.

That's the job for a high viscosity injection epoxy. #1001 High Viscosity Injection Epoxy in the 16.5 oz unit is formulated for filling and sealing wider cracks in concrete foundation walls, and the extra body is the point. It stays in the crack long enough to cure there instead of draining to the bottom of the wall.

Wider, dormant, and dry: that's the pattern. If the crack is wide and wet, keep reading.

Vertical and overhead: pick the resin that stays put

Gravity is the quiet failure mode on wall and ceiling work. A runny resin sags out of the crack before it sets, and you end up with a partial fill and a stain down the wall.

Use a gel. Our #500 Structural Epoxy Gel comes as a two-gallon unit and is made for vertical and overhead concrete crack repair that resists sagging. You get the working time you need without watching your material run down the face. Pair it with #500 Bonding Epoxy when the job calls for structural bonding or repair of concrete and masonry surfaces rather than straight crack injection. #10/35 Anchoring Epoxy covers the related job of setting anchors, bolts and dowels into the same wall.

Active leaks: moisture beats width

The one time width stops being the deciding factor is when the crack is running water. Water-reactive hydrophilic urethane is designed to react with the water in the crack and seal it, which is why it works on leaks where a standard epoxy would be pushed out before it cured. Our #900 Hydrophilic Injection Urethane is built for sealing active leaks in foundation walls.

If you're stopping water movement rather than filling a void, treat it as a different job with different material, and don't expect the same resin to do both.

When injection isn't the job

Not every crack needs a port. Some need protection, not filling. #602 Surface Seal is a surface seal epoxy for sealing cracks and protecting concrete foundation surfaces, and it's the right call when you're managing the face of the wall rather than the void inside it. #551 Polyurethane Rigid Foam handles sealing and insulating foundation and crack repair applications where filling and insulating is the goal.

Knowing when to put the pump away is part of doing the job right.

The field checklist

Before you load the truck or start drilling, run this:

• Width measured at the widest point of the crack.
• Moisture confirmed, dry or actively leaking.
• Movement assessed, and an engineer called if the crack is working.
• Port spacing, injection pressure and cure time taken from the product data sheet, not from memory.
• Surface seal on hand for tight cracks.
• Resin logged with the date and conditions, so the next callback is a lookup instead of an argument.
• One size up and one size down on the shelf, because the crack you measured on Thursday is not always the crack you meet on Monday.

Stock the range, not the guess

There's no single resin that covers every crack, and the jobs that go wrong are usually the ones where somebody tried to make one tube do all of it. Measure first, then pick from the range. Carry the thin, the thick, the gel and the water-reactive, and the job stops being a gamble.

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