If you're shooting in summer daylight with very long exposure times, regardless of whether you damage your sensor or not, you're going to get a completely blown out image, with no recoverable data. If you want very long exposures in bright light, your only real choice is to cut the amount of light going through the lens. For this, you'd normally use a Neutral Density filter of appropriate light-cutting ability.
As with all things photography, the prices of these can vary from a few dollars to many hundreds. When looking at an ND filter, the most important thing to consider is how much light it cuts. See this for information on how ND filters are rated.
There are also variable ND filters of varying quality available. With the cheap ones, using them on a wide-angle lens can result in cross-shaped banding on your image, though this often goes away when shooting at longer focal lengths. The best ones don't suffer from this, but are breathtakingly expensive (a quick search on Amazon shows one selling for $450!).
You can also make a ~10-stop ND filter using welding glass, though these often lend a colour cast to your image, which may or may not be desirable. If you shoot RAW, this is fairly easy to correct (though you'll still lose some colour information).