Using a Primer Shadow as Your Blending Gauge

how to install bathroom moldings

Blending butt joints on our bathroom moldings.  Look for the primer shadow.

[This is part of our How to Install Moldings series.]

When two pieces of molding don’t quite lay flush with each other, you’ll have to do some blending.

This post shows you the trick I use to tell if my moldings are blended perfectly so the joint will not be seen after it’s painted.

And you can use this technique on any kind of molding made from any kind of material, not just flat-stock mdf like in this example.

Primed Surface

This technique requires you have a primed surface.

If you are working with bare wood, then just throw a quick coat of sandable primer on the bare molding or mdf and let it dry. (This is the sandable primer I’ve been using.)

Note: You can’t use a primer like Kilz 2 for this because it does not sand well.

Initial Sanding

Sand the area across the joint.  Since this is mdf flat-stock molding that I’m sanding, I’m using my palm sander to help me make quick work of it.

Sand by hand for smaller or areas or on curved moldings.

how to sand moldings

Look for the Primer Shadow

After sanding back and fort across the joint, take a look to see if there is a primer shadow showing.  If there is, then that joint will show through after the molding is painted.

how to sand moldings

This molding butt joint is not ready because the primer shadow can still be seen.

Below  My left finger is pointing to an area that is blended perfectly because the primer shadow is gone.

My right finger points to an area that still needs work. (Ultimately, I could not blend that right side without taking too much material off.  So I’ll have to live with the results.)

The area on the far left in the corner will have to be sanded by hand.

how to sand moldings

Good on the left and not so good on the right.

Avoid scratching the molding surface by using sandpaper with too large of grit.

To be sure, test your sandpaper grit on a scrap piece of molding before using it on your installed moldings; sandpaper scratches are hard to get out.

I’m using 3M’s P80 sandpaper on these moldings.

how to install and paint moldings

The corner I have to sand by hand.  But this is not difficult.

how to moldings

Most of this butt joint will not show after it’s been properly prepped and painted.

By using this technique and a little patience, you can fix those butt and scarf joints that did not match up perfectly when you installed them.

Good luck!

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