Let's say you want to create the look of text painted on a wooden board and you want the text to look like it's been out in the weather for a long while. There are several methods you can use to distress text. You can find some of them on the web by searching for “GIMP text”, plus one of the following words: weathered, distressed, worn.
This tutorial describes how to create the following image using a photo of a gray board, some text, and a photo of a rusty metal texture that is processed and turned into a layer mask. You can download the two photos (Weathered Gray Wood 0099 and Rusty Metal 0817) from the Free Images page of my blog.
Step 1 – Create Text on Top of a Board
Load a photo of a wooden board of a decent size into GIMP. Orient it and crop it to taste. Name the layer “board”. Set the Foreground color to white  and select the Text tool . Choose an interesting font  of a good size  and type some text over the board. Change the Mode of the text layer to Overlay, then use the Move tool  to place the text where you want it, or the Alignment tool  to center it.
* TIP *
If you choose a typeface that is not vertically centered in its box, then the Alignment tool won't be able to center it vertically. To deal with this, create a new transparent layer underneath the text layer, then right-click on the text layer and choose Merge Down. Use the Rectangle Select tool to create a selection box around the text, putting each side of the selection box two pixels off from the edge of the letters to make absolutely sure you don't cut off any of the lettering. Zoom in close so you can see what you're doing. Then choose Layer > Crop to Selection, then remove the selection (CTRL+SHFT+A). Now you can use the Alignment tool to center the text vertically.
Step 2 – Add a Rusty Metal Texture
Load a rusty metal texture into GIMP on a separate canvas (not over the board image). Use the Rectangle Select tool  to select a likely looking section, one that has an interesting mixture of light and dark areas.
Copy the selection (CTRL+C), then go back to the board image and paste the copy (CTRL+V). Double-click on the Floating Selection (Pasted Layer) that appears and name the layer “rusty metal”. In this case you'll want the texture to go in the same direction as the board's texture, so use Layer > Transform > Rotate 90° Clockwise to rotate the layer. Choose Layer > Scale Layer, click on the link to disconnect the width and height, then enter the dimensions of your canvas. The rusty metal texture should now cover the canvas. If it's out of position, use the Move tool to adjust it.
Step 3 – Prep the Rusty Metal Texture for Becoming a Mask
With the “rusty metal” layer active, choose Colors > Desaturate and pick one of the shades. Luminosity tends to look pretty good. Then choose Colors > Threshold and slide the black triangle until you get a good mixture of black and white. The black areas are where the letters are going to appear rubbed off. You can skip the desaturation step and just use Threshold if you like. This will give you a slightly different, rougher-looking texture.
Step 4 – Add a Mask to the Text Layer
Copy the black and white texture layer, then click the eyeball to hide it . Right-click on the text layer and choose Add Layer Mask . You can initialize the layer to any of the options because you'll be overwriting it.
With the mask selected , paste the copy (CTRL+V). Right-click on the Floating Selection (Pasted Layer) and choose Anchor Layer . This will put the black and white texture into the mask for the text layer.
You should see that the letters look worn now. If the white text looks too faint on the wood, duplicate the text layer .
Et voila! Weathered letters on a wooden board in a few easy steps. If you have another technique that accomplishes a similar effect, please share it in a comment. I love learning new things.